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  <div class="section" id="live-block-device-operations">
<h1><a class="toc-backref" href="#id2">Live Block Device Operations</a><a class="headerlink" href="#live-block-device-operations" title="Permalink to this headline">¶</a></h1>
<p>QEMU Block Layer currently (as of QEMU 2.9) supports four major kinds of
live block device jobs – stream, commit, mirror, and backup.  These can
be used to manipulate disk image chains to accomplish certain tasks,
namely: live copy data from backing files into overlays; shorten long
disk image chains by merging data from overlays into backing files; live
synchronize data from a disk image chain (including current active disk)
to another target image; and point-in-time (and incremental) backups of
a block device.  Below is a description of the said block (QMP)
primitives, and some (non-exhaustive list of) examples to illustrate
their use.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The file <code class="docutils literal notranslate"><span class="pre">qapi/block-core.json</span></code> in the QEMU source tree has the
canonical QEMU API (QAPI) schema documentation for the QMP
primitives discussed here.</p>
</div>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#live-block-device-operations" id="id2">Live Block Device Operations</a><ul>
<li><a class="reference internal" href="#disk-image-backing-chain-notation" id="id3">Disk image backing chain notation</a></li>
<li><a class="reference internal" href="#brief-overview-of-live-block-qmp-primitives" id="id4">Brief overview of live block QMP primitives</a></li>
<li><a class="reference internal" href="#interacting-with-a-qemu-instance" id="id5">Interacting with a QEMU instance</a></li>
<li><a class="reference internal" href="#example-disk-image-chain" id="id6">Example disk image chain</a></li>
<li><a class="reference internal" href="#a-note-on-points-in-time-vs-file-names" id="id7">A note on points-in-time vs file names</a></li>
<li><a class="reference internal" href="#live-block-streaming-block-stream" id="id8">Live block streaming — <code class="docutils literal notranslate"><span class="pre">block-stream</span></code></a><ul>
<li><a class="reference internal" href="#qmp-invocation-for-block-stream" id="id9">QMP invocation for <code class="docutils literal notranslate"><span class="pre">block-stream</span></code></a></li>
</ul>
</li>
<li><a class="reference internal" href="#live-block-commit-block-commit" id="id10">Live block commit — <code class="docutils literal notranslate"><span class="pre">block-commit</span></code></a><ul>
<li><a class="reference internal" href="#qmp-invocation-for-block-commit" id="id11">QMP invocation for <code class="docutils literal notranslate"><span class="pre">block-commit</span></code></a></li>
</ul>
</li>
<li><a class="reference internal" href="#live-disk-synchronization-drive-mirror-and-blockdev-mirror" id="id12">Live disk synchronization — <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> and <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a><ul>
<li><a class="reference internal" href="#qmp-invocation-for-drive-mirror" id="id13">QMP invocation for <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code></a></li>
<li><a class="reference internal" href="#qmp-invocation-for-live-storage-migration-with-drive-mirror-nbd" id="id14">QMP invocation for live storage migration with <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> + NBD</a></li>
<li><a class="reference internal" href="#notes-on-blockdev-mirror" id="id15">Notes on <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a></li>
<li><a class="reference internal" href="#qmp-invocation-for-blockdev-mirror" id="id16">QMP invocation for <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a></li>
</ul>
</li>
<li><a class="reference internal" href="#live-disk-backup-drive-backup-and-blockdev-backup" id="id17">Live disk backup — <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> and <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a><ul>
<li><a class="reference internal" href="#qmp-invocation-for-drive-backup" id="id18">QMP invocation for <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code></a></li>
<li><a class="reference internal" href="#notes-on-blockdev-backup" id="id19">Notes on <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a></li>
<li><a class="reference internal" href="#qmp-invocation-for-blockdev-backup" id="id20">QMP invocation for <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
<div class="section" id="disk-image-backing-chain-notation">
<h2><a class="toc-backref" href="#id3">Disk image backing chain notation</a><a class="headerlink" href="#disk-image-backing-chain-notation" title="Permalink to this headline">¶</a></h2>
<p>A simple disk image chain.  (This can be created live using QMP
<code class="docutils literal notranslate"><span class="pre">blockdev-snapshot-sync</span></code>, or offline via <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code>):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>               <span class="p">(</span><span class="n">Live</span> <span class="n">QEMU</span><span class="p">)</span>
                    <span class="o">|</span>
                    <span class="o">.</span>
                    <span class="n">V</span>

        <span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;-----</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span>

<span class="p">(</span><span class="n">backing</span> <span class="n">file</span><span class="p">)</span>    <span class="p">(</span><span class="n">overlay</span><span class="p">)</span>
</pre></div>
</div>
<p>The arrow can be read as: Image [A] is the backing file of disk image
[B].  And live QEMU is currently writing to image [B], consequently, it
is also referred to as the “active layer”.</p>
<p>There are two kinds of terminology that are common when referring to
files in a disk image backing chain:</p>
<ol class="arabic simple">
<li>Directional: ‘base’ and ‘top’.  Given the simple disk image chain
above, image [A] can be referred to as ‘base’, and image [B] as
‘top’.  (This terminology can be seen in in QAPI schema file,
block-core.json.)</li>
<li>Relational: ‘backing file’ and ‘overlay’.  Again, taking the same
simple disk image chain from the above, disk image [A] is referred
to as the backing file, and image [B] as overlay.</li>
</ol>
<blockquote>
<div>Throughout this document, we will use the relational terminology.</div></blockquote>
<div class="admonition important">
<p class="first admonition-title">Important</p>
<p class="last">The overlay files can generally be any format that supports a
backing file, although QCOW2 is the preferred format and the one
used in this document.</p>
</div>
</div>
<div class="section" id="brief-overview-of-live-block-qmp-primitives">
<h2><a class="toc-backref" href="#id4">Brief overview of live block QMP primitives</a><a class="headerlink" href="#brief-overview-of-live-block-qmp-primitives" title="Permalink to this headline">¶</a></h2>
<p>The following are the four different kinds of live block operations that
QEMU block layer supports.</p>
<ol class="arabic">
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">block-stream</span></code>: Live copy of data from backing files into overlay
files.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>Once the ‘stream’ operation has finished, three things to
note:</p>
<blockquote class="last">
<div><ol class="loweralpha simple">
<li>QEMU rewrites the backing chain to remove
reference to the now-streamed and redundant backing
file;</li>
<li>the streamed file <em>itself</em> won’t be removed by QEMU,
and must be explicitly discarded by the user;</li>
<li>the streamed file remains valid – i.e. further
overlays can be created based on it.  Refer the
<code class="docutils literal notranslate"><span class="pre">block-stream</span></code> section further below for more
details.</li>
</ol>
</div></blockquote>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">block-commit</span></code>: Live merge of data from overlay files into backing
files (with the optional goal of removing the overlay file from the
chain).  Since QEMU 2.0, this includes “active <code class="docutils literal notranslate"><span class="pre">block-commit</span></code>”
(i.e. merge the current active layer into the base image).</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>Once the ‘commit’ operation has finished, there are three
things to note here as well:</p>
<blockquote class="last">
<div><ol class="loweralpha simple">
<li>QEMU rewrites the backing chain to remove reference
to now-redundant overlay images that have been
committed into a backing file;</li>
<li>the committed file <em>itself</em> won’t be removed by QEMU
– it ought to be manually removed;</li>
<li>however, unlike in the case of <code class="docutils literal notranslate"><span class="pre">block-stream</span></code>, the
intermediate images will be rendered invalid – i.e.
no more further overlays can be created based on
them.  Refer the <code class="docutils literal notranslate"><span class="pre">block-commit</span></code> section further
below for more details.</li>
</ol>
</div></blockquote>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> (and <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code>): Synchronize a running
disk to another image.</p>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> (and <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code>): Point-in-time (live) copy
of a block device to a destination.</p>
</li>
</ol>
</div>
<div class="section" id="interacting-with-a-qemu-instance">
<span id="id1"></span><h2><a class="toc-backref" href="#id5">Interacting with a QEMU instance</a><a class="headerlink" href="#interacting-with-a-qemu-instance" title="Permalink to this headline">¶</a></h2>
<p>To show some example invocations of command-line, we will use the
following invocation of QEMU, with a QMP server running over UNIX
socket:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ ./x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
    -M q35 -nodefaults -m 512 \
    -blockdev node-name=node-A,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./a.qcow2 \
    -device virtio-blk,drive=node-A,id=virtio0 \
    -monitor stdio -qmp unix:/tmp/qmp-sock,server,nowait
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">-blockdev</span></code> command-line option, used above, is available from
QEMU 2.9 onwards.  In the above invocation, notice the <code class="docutils literal notranslate"><span class="pre">node-name</span></code>
parameter that is used to refer to the disk image a.qcow2 (‘node-A’) –
this is a cleaner way to refer to a disk image (as opposed to referring
to it by spelling out file paths).  So, we will continue to designate a
<code class="docutils literal notranslate"><span class="pre">node-name</span></code> to each further disk image created (either via
<code class="docutils literal notranslate"><span class="pre">blockdev-snapshot-sync</span></code>, or <code class="docutils literal notranslate"><span class="pre">blockdev-add</span></code>) as part of the disk
image chain, and continue to refer to the disks using their
<code class="docutils literal notranslate"><span class="pre">node-name</span></code> (where possible, because <code class="docutils literal notranslate"><span class="pre">block-commit</span></code> does not yet, as
of QEMU 2.9, accept <code class="docutils literal notranslate"><span class="pre">node-name</span></code> parameter) when performing various
block operations.</p>
<p>To interact with the QEMU instance launched above, we will use the
<code class="docutils literal notranslate"><span class="pre">qmp-shell</span></code> utility (located at: <code class="docutils literal notranslate"><span class="pre">qemu/scripts/qmp</span></code>, as part of the
QEMU source directory), which takes key-value pairs for QMP commands.
Invoke it as below (which will also print out the complete raw JSON
syntax for reference – examples in the following sections):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ ./qmp-shell -v -p /tmp/qmp-sock
(QEMU)
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">In the event we have to repeat a certain QMP command, we will: for
the first occurrence of it, show the <code class="docutils literal notranslate"><span class="pre">qmp-shell</span></code> invocation, <em>and</em>
the corresponding raw JSON QMP syntax; but for subsequent
invocations, present just the <code class="docutils literal notranslate"><span class="pre">qmp-shell</span></code> syntax, and omit the
equivalent JSON output.</p>
</div>
</div>
<div class="section" id="example-disk-image-chain">
<h2><a class="toc-backref" href="#id6">Example disk image chain</a><a class="headerlink" href="#example-disk-image-chain" title="Permalink to this headline">¶</a></h2>
<p>We will use the below disk image chain (and occasionally spelling it
out where appropriate) when discussing various primitives:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>Where [A] is the original base image; [B] and [C] are intermediate
overlay images; image [D] is the active layer – i.e. live QEMU is
writing to it.  (The rule of thumb is: live QEMU will always be pointing
to the rightmost image in a disk image chain.)</p>
<p>The above image chain can be created by invoking
<code class="docutils literal notranslate"><span class="pre">blockdev-snapshot-sync</span></code> commands as following (which shows the
creation of overlay image [B]) using the <code class="docutils literal notranslate"><span class="pre">qmp-shell</span></code> (our invocation
also prints the raw JSON invocation of it):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">A</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">b</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;blockdev-snapshot-sync&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;node-name&quot;</span><span class="p">:</span> <span class="s2">&quot;node-A&quot;</span><span class="p">,</span>
        <span class="s2">&quot;snapshot-file&quot;</span><span class="p">:</span> <span class="s2">&quot;b.qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;format&quot;</span><span class="p">:</span> <span class="s2">&quot;qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;snapshot-node-name&quot;</span><span class="p">:</span> <span class="s2">&quot;node-B&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Here, “node-A” is the name QEMU internally uses to refer to the base
image [A] – it is the backing file, based on which the overlay image,
[B], is created.</p>
<p>To create the rest of the overlay images, [C], and [D] (omitting the raw
JSON output for brevity):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">c</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">C</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">C</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">d</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
</pre></div>
</div>
</div>
<div class="section" id="a-note-on-points-in-time-vs-file-names">
<h2><a class="toc-backref" href="#id7">A note on points-in-time vs file names</a><a class="headerlink" href="#a-note-on-points-in-time-vs-file-names" title="Permalink to this headline">¶</a></h2>
<p>In our disk image chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>We have <em>three</em> points in time and an active layer:</p>
<ul class="simple">
<li>Point 1: Guest state when [B] was created is contained in file [A]</li>
<li>Point 2: Guest state when [C] was created is contained in [A] + [B]</li>
<li>Point 3: Guest state when [D] was created is contained in
[A] + [B] + [C]</li>
<li>Active layer: Current guest state is contained in [A] + [B] + [C] +
[D]</li>
</ul>
<p>Therefore, be aware with naming choices:</p>
<ul class="simple">
<li>Naming a file after the time it is created is misleading – the
guest data for that point in time is <em>not</em> contained in that file
(as explained earlier)</li>
<li>Rather, think of files as a <em>delta</em> from the backing file</li>
</ul>
</div>
<div class="section" id="live-block-streaming-block-stream">
<h2><a class="toc-backref" href="#id8">Live block streaming — <code class="docutils literal notranslate"><span class="pre">block-stream</span></code></a><a class="headerlink" href="#live-block-streaming-block-stream" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">block-stream</span></code> command allows you to do live copy data from backing
files into overlay images.</p>
<p>Given our original example disk image chain from earlier:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>The disk image chain can be shortened in one of the following different
ways (not an exhaustive list).</p>
<ol class="arabic" id="case-1">
<li><p class="first">Merge everything into the active layer: I.e. copy all contents from
the base image, [A], and overlay images, [B] and [C], into [D],
<em>while</em> the guest is running.  The resulting chain will be a
standalone image, [D] – with contents from [A], [B] and [C] merged
into it (where live QEMU writes go to):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
</li>
</ol>
<ol class="arabic" id="case-2" start="2">
<li><p class="first">Taking the same example disk image chain mentioned earlier, merge
only images [B] and [C] into [D], the active layer.  The result will
be contents of images [B] and [C] will be copied into [D], and the
backing file pointer of image [D] will be adjusted to point to image
[A].  The resulting chain will be:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
</li>
</ol>
<ol class="arabic" id="case-3" start="3">
<li><p class="first">Intermediate streaming (available since QEMU 2.8): Starting afresh
with the original example disk image chain, with a total of four
images, it is possible to copy contents from image [B] into image
[C].  Once the copy is finished, image [B] can now be (optionally)
discarded; and the backing file pointer of image [C] will be
adjusted to point to [A].  I.e. after performing “intermediate
streaming” of [B] into [C], the resulting image chain will be (where
live QEMU is writing to [D]):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
</li>
</ol>
<div class="section" id="qmp-invocation-for-block-stream">
<h3><a class="toc-backref" href="#id9">QMP invocation for <code class="docutils literal notranslate"><span class="pre">block-stream</span></code></a><a class="headerlink" href="#qmp-invocation-for-block-stream" title="Permalink to this headline">¶</a></h3>
<p>For <a class="reference internal" href="#case-1">Case-1</a>, to merge contents of all the backing files into the
active layer, where ‘node-D’ is the current active image (by default
<code class="docutils literal notranslate"><span class="pre">block-stream</span></code> will flatten the entire chain); <code class="docutils literal notranslate"><span class="pre">qmp-shell</span></code> (and its
corresponding JSON output):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">stream</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-stream&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>For <a class="reference internal" href="#case-2">Case-2</a>, merge contents of the images [B] and [C] into [D], where
image [D] ends up referring to image [A] as its backing file:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">stream</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">base</span><span class="o">-</span><span class="n">node</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">A</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
</pre></div>
</div>
<p>And for <a class="reference internal" href="#case-3">Case-3</a>, of “intermediate” streaming”, merge contents of
images [B] into [C], where [C] ends up referring to [A] as its backing
image:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">stream</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">C</span> <span class="n">base</span><span class="o">-</span><span class="n">node</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">A</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
</pre></div>
</div>
<p>Progress of a <code class="docutils literal notranslate"><span class="pre">block-stream</span></code> operation can be monitored via the QMP
command:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">block</span><span class="o">-</span><span class="n">jobs</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;query-block-jobs&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Once the <code class="docutils literal notranslate"><span class="pre">block-stream</span></code> operation has completed, QEMU will emit an
event, <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code>.  The intermediate overlays remain valid,
and can now be (optionally) discarded, or retained to create further
overlays based on them.  Finally, the <code class="docutils literal notranslate"><span class="pre">block-stream</span></code> jobs can be
restarted at anytime.</p>
</div>
</div>
<div class="section" id="live-block-commit-block-commit">
<h2><a class="toc-backref" href="#id10">Live block commit — <code class="docutils literal notranslate"><span class="pre">block-commit</span></code></a><a class="headerlink" href="#live-block-commit-block-commit" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">block-commit</span></code> command lets you merge live data from overlay
images into backing file(s).  Since QEMU 2.0, this includes “live active
commit” (i.e. it is possible to merge the “active layer”, the right-most
image in a disk image chain where live QEMU will be writing to, into the
base image).  This is analogous to <code class="docutils literal notranslate"><span class="pre">block-stream</span></code>, but in the opposite
direction.</p>
<p>Again, starting afresh with our example disk image chain, where live
QEMU is writing to the right-most image in the chain, [D]:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>The disk image chain can be shortened in one of the following ways:</p>
<ol class="arabic" id="block-commit-case-1">
<li><p class="first">Commit content from only image [B] into image [A].  The resulting
chain is the following, where image [C] is adjusted to point at [A]
as its new backing file:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">Commit content from images [B] and [C] into image [A].  The
resulting chain, where image [D] is adjusted to point to image [A]
as its new backing file:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
</li>
</ol>
<ol class="arabic" id="block-commit-case-3" start="3">
<li><p class="first">Commit content from images [B], [C], and the active layer [D] into
image [A].  The resulting chain (in this case, a consolidated single
image):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">Commit content from image only image [C] into image [B].  The
resulting chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">Commit content from image [C] and the active layer [D] into image
[B].  The resulting chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span>
</pre></div>
</div>
</li>
</ol>
<div class="section" id="qmp-invocation-for-block-commit">
<h3><a class="toc-backref" href="#id11">QMP invocation for <code class="docutils literal notranslate"><span class="pre">block-commit</span></code></a><a class="headerlink" href="#qmp-invocation-for-block-commit" title="Permalink to this headline">¶</a></h3>
<p>For <a class="reference internal" href="#block-commit-case-1"><span class="std std-ref">Case-1</span></a>, to merge contents only from
image [B] into image [A], the invocation is as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">commit</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">base</span><span class="o">=</span><span class="n">a</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">top</span><span class="o">=</span><span class="n">b</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-commit&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;top&quot;</span><span class="p">:</span> <span class="s2">&quot;b.qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;base&quot;</span><span class="p">:</span> <span class="s2">&quot;a.qcow2&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Once the above <code class="docutils literal notranslate"><span class="pre">block-commit</span></code> operation has completed, a
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code> event will be issued, and no further action is
required.  As the end result, the backing file of image [C] is adjusted
to point to image [A], and the original 4-image chain will end up being
transformed to:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>The intermediate image [B] is invalid (as in: no more further
overlays based on it can be created).</p>
<p class="last">Reasoning: An intermediate image after a ‘stream’ operation still
represents that old point-in-time, and may be valid in that context.
However, an intermediate image after a ‘commit’ operation no longer
represents any point-in-time, and is invalid in any context.</p>
</div>
<p>However, <a class="reference internal" href="#block-commit-case-3"><span class="std std-ref">Case-3</span></a> (also called: “active
<code class="docutils literal notranslate"><span class="pre">block-commit</span></code>”) is a <em>two-phase</em> operation: In the first phase, the
content from the active overlay, along with the intermediate overlays,
is copied into the backing file (also called the base image).  In the
second phase, adjust the said backing file as the current active image
– possible via issuing the command <code class="docutils literal notranslate"><span class="pre">block-job-complete</span></code>.  Optionally,
the <code class="docutils literal notranslate"><span class="pre">block-commit</span></code> operation can be cancelled by issuing the command
<code class="docutils literal notranslate"><span class="pre">block-job-cancel</span></code>, but be careful when doing this.</p>
<p>Once the <code class="docutils literal notranslate"><span class="pre">block-commit</span></code> operation has completed, the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code> will be emitted, signalling that the synchronization
has finished.  Now the job can be gracefully completed by issuing the
command <code class="docutils literal notranslate"><span class="pre">block-job-complete</span></code> – until such a command is issued, the
‘commit’ operation remains active.</p>
<p>The following is the flow for <a class="reference internal" href="#block-commit-case-3"><span class="std std-ref">Case-3</span></a> to
convert a disk image chain such as this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>Into:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span>
</pre></div>
</div>
<p>Where content from all the subsequent overlays, [B], and [C], including
the active layer, [D], is committed back to [A] – which is where live
QEMU is performing all its current writes).</p>
<p>Start the “active <code class="docutils literal notranslate"><span class="pre">block-commit</span></code>” operation:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">commit</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">base</span><span class="o">=</span><span class="n">a</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">top</span><span class="o">=</span><span class="n">d</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-commit&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;top&quot;</span><span class="p">:</span> <span class="s2">&quot;d.qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;base&quot;</span><span class="p">:</span> <span class="s2">&quot;a.qcow2&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Once the synchronization has completed, the event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code> will
be emitted.</p>
<p>Then, optionally query for the status of the active block operations.
We can see the ‘commit’ job is now ready to be completed, as indicated
by the line <em>“ready”: true</em>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">block</span><span class="o">-</span><span class="n">jobs</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;query-block-jobs&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;return&quot;</span><span class="p">:</span> <span class="p">[</span>
        <span class="p">{</span>
            <span class="s2">&quot;busy&quot;</span><span class="p">:</span> <span class="n">false</span><span class="p">,</span>
            <span class="s2">&quot;type&quot;</span><span class="p">:</span> <span class="s2">&quot;commit&quot;</span><span class="p">,</span>
            <span class="s2">&quot;len&quot;</span><span class="p">:</span> <span class="mi">1376256</span><span class="p">,</span>
            <span class="s2">&quot;paused&quot;</span><span class="p">:</span> <span class="n">false</span><span class="p">,</span>
            <span class="s2">&quot;ready&quot;</span><span class="p">:</span> <span class="n">true</span><span class="p">,</span>
            <span class="s2">&quot;io-status&quot;</span><span class="p">:</span> <span class="s2">&quot;ok&quot;</span><span class="p">,</span>
            <span class="s2">&quot;offset&quot;</span><span class="p">:</span> <span class="mi">1376256</span><span class="p">,</span>
            <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
            <span class="s2">&quot;speed&quot;</span><span class="p">:</span> <span class="mi">0</span>
        <span class="p">}</span>
    <span class="p">]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Gracefully complete the ‘commit’ block device job:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">job</span><span class="o">-</span><span class="n">complete</span> <span class="n">device</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-job-complete&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;return&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Finally, once the above job is completed, an event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code> will be emitted.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The invocation for rest of the cases (2, 4, and 5), discussed in the
previous section, is omitted for brevity.</p>
</div>
</div>
</div>
<div class="section" id="live-disk-synchronization-drive-mirror-and-blockdev-mirror">
<h2><a class="toc-backref" href="#id12">Live disk synchronization — <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> and <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a><a class="headerlink" href="#live-disk-synchronization-drive-mirror-and-blockdev-mirror" title="Permalink to this headline">¶</a></h2>
<p>Synchronize a running disk image chain (all or part of it) to a target
image.</p>
<p>Again, given our familiar disk image chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> (and its newer equivalent <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code>)
allows you to copy data from the entire chain into a single target image
(which can be located on a different host), [E].</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>When you cancel an in-progress ‘mirror’ job <em>before</em> the source and
target are synchronized, <code class="docutils literal notranslate"><span class="pre">block-job-cancel</span></code> will emit the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_CANCELLED</span></code>.  However, note that if you cancel a
‘mirror’ job <em>after</em> it has indicated (via the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code>) that the source and target have reached
synchronization, then the event emitted by <code class="docutils literal notranslate"><span class="pre">block-job-cancel</span></code>
changes to <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code>.</p>
<p class="last">Besides the ‘mirror’ job, the “active <code class="docutils literal notranslate"><span class="pre">block-commit</span></code>” is the only
other block device job that emits the event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code>.
The rest of the block device jobs (‘stream’, “non-active
<code class="docutils literal notranslate"><span class="pre">block-commit</span></code>”, and ‘backup’) end automatically.</p>
</div>
<p>So there are two possible actions to take, after a ‘mirror’ job has
emitted the event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code>, indicating that the source and
target have reached synchronization:</p>
<ol class="arabic simple">
<li>Issuing the command <code class="docutils literal notranslate"><span class="pre">block-job-cancel</span></code> (after it emits the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code>) will create a point-in-time (which is at
the time of <em>triggering</em> the cancel command) copy of the entire disk
image chain (or only the top-most image, depending on the <code class="docutils literal notranslate"><span class="pre">sync</span></code>
mode), contained in the target image [E]. One use case for this is
live VM migration with non-shared storage.</li>
<li>Issuing the command <code class="docutils literal notranslate"><span class="pre">block-job-complete</span></code> (after it emits the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code>) will adjust the guest device (i.e. live
QEMU) to point to the target image, [E], causing all the new writes
from this point on to happen there.</li>
</ol>
<p>About synchronization modes: The synchronization mode determines
<em>which</em> part of the disk image chain will be copied to the target.
Currently, there are four different kinds:</p>
<ol class="arabic">
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">full</span></code> – Synchronize the content of entire disk image chain to
the target</p>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">top</span></code> – Synchronize only the contents of the top-most disk image
in the chain to the target</p>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">none</span></code> – Synchronize only the new writes from this point on.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">In the case of <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> (or <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code>),
the behavior of <code class="docutils literal notranslate"><span class="pre">none</span></code> synchronization mode is different.
Normally, a <code class="docutils literal notranslate"><span class="pre">backup</span></code> job consists of two parts: Anything
that is overwritten by the guest is first copied out to
the backup, and in the background the whole image is
copied from start to end. With <code class="docutils literal notranslate"><span class="pre">sync=none</span></code>, it’s only
the first part.</p>
</div>
</li>
<li><p class="first"><code class="docutils literal notranslate"><span class="pre">incremental</span></code> – Synchronize content that is described by the
dirty bitmap</p>
</li>
</ol>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Refer to the <a class="reference internal" href="bitmaps.html"><span class="doc">Dirty Bitmaps and Incremental Backup</span></a> document in the QEMU source
tree to learn about the detailed workings of the <code class="docutils literal notranslate"><span class="pre">incremental</span></code>
synchronization mode.</p>
</div>
<div class="section" id="qmp-invocation-for-drive-mirror">
<h3><a class="toc-backref" href="#id13">QMP invocation for <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code></a><a class="headerlink" href="#qmp-invocation-for-drive-mirror" title="Permalink to this headline">¶</a></h3>
<p>To copy the contents of the entire disk image chain, from [A] all the
way to [D], to a new target (<code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> will create the destination
file, if it doesn’t already exist), call it [E]:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">drive</span><span class="o">-</span><span class="n">mirror</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">target</span><span class="o">=</span><span class="n">e</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">sync</span><span class="o">=</span><span class="n">full</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;drive-mirror&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;target&quot;</span><span class="p">:</span> <span class="s2">&quot;e.qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;sync&quot;</span><span class="p">:</span> <span class="s2">&quot;full&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">&quot;sync&quot;:</span> <span class="pre">&quot;full&quot;</span></code>, from the above, means: copy the <em>entire</em> chain
to the destination.</p>
<p>Following the above, querying for active block jobs will show that a
‘mirror’ job is “ready” to be completed (and QEMU will also emit an
event, <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code>):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">block</span><span class="o">-</span><span class="n">jobs</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;query-block-jobs&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;return&quot;</span><span class="p">:</span> <span class="p">[</span>
        <span class="p">{</span>
            <span class="s2">&quot;busy&quot;</span><span class="p">:</span> <span class="n">false</span><span class="p">,</span>
            <span class="s2">&quot;type&quot;</span><span class="p">:</span> <span class="s2">&quot;mirror&quot;</span><span class="p">,</span>
            <span class="s2">&quot;len&quot;</span><span class="p">:</span> <span class="mi">21757952</span><span class="p">,</span>
            <span class="s2">&quot;paused&quot;</span><span class="p">:</span> <span class="n">false</span><span class="p">,</span>
            <span class="s2">&quot;ready&quot;</span><span class="p">:</span> <span class="n">true</span><span class="p">,</span>
            <span class="s2">&quot;io-status&quot;</span><span class="p">:</span> <span class="s2">&quot;ok&quot;</span><span class="p">,</span>
            <span class="s2">&quot;offset&quot;</span><span class="p">:</span> <span class="mi">21757952</span><span class="p">,</span>
            <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
            <span class="s2">&quot;speed&quot;</span><span class="p">:</span> <span class="mi">0</span>
        <span class="p">}</span>
    <span class="p">]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>And, as noted in the previous section, there are two possible actions
at this point:</p>
<ol class="loweralpha">
<li><p class="first">Create a point-in-time snapshot by ending the synchronization.  The
point-in-time is at the time of <em>ending</em> the sync.  (The result of
the following being: the target image, [E], will be populated with
content from the entire chain, [A] to [D]):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">job</span><span class="o">-</span><span class="n">cancel</span> <span class="n">device</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-job-cancel&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">Or, complete the operation and pivot the live QEMU to the target
copy:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">job</span><span class="o">-</span><span class="n">complete</span> <span class="n">device</span><span class="o">=</span><span class="n">job0</span>
</pre></div>
</div>
</li>
</ol>
<p>In either of the above cases, if you once again run the
<cite>query-block-jobs</cite> command, there should not be any active block
operation.</p>
<p>Comparing ‘commit’ and ‘mirror’: In both then cases, the overlay images
can be discarded.  However, with ‘commit’, the <em>existing</em> base image
will be modified (by updating it with contents from overlays); while in
the case of ‘mirror’, a <em>new</em> target image is populated with the data
from the disk image chain.</p>
</div>
<div class="section" id="qmp-invocation-for-live-storage-migration-with-drive-mirror-nbd">
<h3><a class="toc-backref" href="#id14">QMP invocation for live storage migration with <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> + NBD</a><a class="headerlink" href="#qmp-invocation-for-live-storage-migration-with-drive-mirror-nbd" title="Permalink to this headline">¶</a></h3>
<p>Live storage migration (without shared storage setup) is one of the most
common use-cases that takes advantage of the <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> primitive
and QEMU’s built-in Network Block Device (NBD) server.  Here’s a quick
walk-through of this setup.</p>
<p>Given the disk image chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>Instead of copying content from the entire chain, synchronize <em>only</em> the
contents of the <em>top</em>-most disk image (i.e. the active layer), [D], to a
target, say, [TargetDisk].</p>
<div class="admonition important">
<p class="first admonition-title">Important</p>
<p class="last">The destination host must already have the contents of the backing
chain, involving images [A], [B], and [C], visible via other means
– whether by <code class="docutils literal notranslate"><span class="pre">cp</span></code>, <code class="docutils literal notranslate"><span class="pre">rsync</span></code>, or by some storage array-specific
command.)</p>
</div>
<p>Sometimes, this is also referred to as “shallow copy” – because only
the “active layer”, and not the rest of the image chain, is copied to
the destination.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">In this example, for the sake of simplicity, we’ll be using the same
<code class="docutils literal notranslate"><span class="pre">localhost</span></code> as both source and destination.</p>
</div>
<p>As noted earlier, on the destination host the contents of the backing
chain – from images [A] to [C] – are already expected to exist in some
form (e.g. in a file called, <code class="docutils literal notranslate"><span class="pre">Contents-of-A-B-C.qcow2</span></code>).  Now, on the
destination host, let’s create a target overlay image (with the image
<code class="docutils literal notranslate"><span class="pre">Contents-of-A-B-C.qcow2</span></code> as its backing file), to which the contents
of image [D] (from the source QEMU) will be mirrored to:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-img create -f qcow2 -b ./Contents-of-A-B-C.qcow2 \
    -F qcow2 ./target-disk.qcow2
</pre></div>
</div>
<p>And start the destination QEMU (we already have the source QEMU running
– discussed in the section: <a class="reference internal" href="#interacting-with-a-qemu-instance">Interacting with a QEMU instance</a>)
instance, with the following invocation.  (As noted earlier, for
simplicity’s sake, the destination QEMU is started on the same host, but
it could be located elsewhere):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ ./x86_64-softmmu/qemu-system-x86_64 -display none -no-user-config \
    -M q35 -nodefaults -m 512 \
    -blockdev node-name=node-TargetDisk,driver=qcow2,file.driver=file,file.node-name=file,file.filename=./target-disk.qcow2 \
    -device virtio-blk,drive=node-TargetDisk,id=virtio0 \
    -S -monitor stdio -qmp unix:./qmp-sock2,server,nowait \
    -incoming tcp:localhost:6666
</pre></div>
</div>
<p>Given the disk image chain on source QEMU:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>On the destination host, it is expected that the contents of the chain
<code class="docutils literal notranslate"><span class="pre">[A]</span> <span class="pre">&lt;--</span> <span class="pre">[B]</span> <span class="pre">&lt;--</span> <span class="pre">[C]</span></code> are <em>already</em> present, and therefore copy <em>only</em>
the content of image [D].</p>
<ol class="arabic">
<li><p class="first">[On <em>destination</em> QEMU] As part of the first step, start the
built-in NBD server on a given host (local host, represented by
<code class="docutils literal notranslate"><span class="pre">::</span></code>)and port:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">nbd</span><span class="o">-</span><span class="n">server</span><span class="o">-</span><span class="n">start</span> <span class="n">addr</span><span class="o">=</span><span class="p">{</span><span class="s2">&quot;type&quot;</span><span class="p">:</span><span class="s2">&quot;inet&quot;</span><span class="p">,</span><span class="s2">&quot;data&quot;</span><span class="p">:{</span><span class="s2">&quot;host&quot;</span><span class="p">:</span><span class="s2">&quot;::&quot;</span><span class="p">,</span><span class="s2">&quot;port&quot;</span><span class="p">:</span><span class="s2">&quot;49153&quot;</span><span class="p">}}</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;nbd-server-start&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;addr&quot;</span><span class="p">:</span> <span class="p">{</span>
            <span class="s2">&quot;data&quot;</span><span class="p">:</span> <span class="p">{</span>
                <span class="s2">&quot;host&quot;</span><span class="p">:</span> <span class="s2">&quot;::&quot;</span><span class="p">,</span>
                <span class="s2">&quot;port&quot;</span><span class="p">:</span> <span class="s2">&quot;49153&quot;</span>
            <span class="p">},</span>
            <span class="s2">&quot;type&quot;</span><span class="p">:</span> <span class="s2">&quot;inet&quot;</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">[On <em>destination</em> QEMU] And export the destination disk image using
QEMU’s built-in NBD server:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">nbd</span><span class="o">-</span><span class="n">server</span><span class="o">-</span><span class="n">add</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">TargetDisk</span> <span class="n">writable</span><span class="o">=</span><span class="n">true</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;nbd-server-add&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-TargetDisk&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">[On <em>source</em> QEMU] Then, invoke <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> (NB: since we’re
running <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> with <code class="docutils literal notranslate"><span class="pre">mode=existing</span></code> (meaning:
synchronize to a pre-created file, therefore ‘existing’, file on the
target host), with the synchronization mode as ‘top’ (<code class="docutils literal notranslate"><span class="pre">&quot;sync:</span>
<span class="pre">&quot;top&quot;</span></code>):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">drive</span><span class="o">-</span><span class="n">mirror</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">target</span><span class="o">=</span><span class="n">nbd</span><span class="p">:</span><span class="n">localhost</span><span class="p">:</span><span class="mi">49153</span><span class="p">:</span><span class="n">exportname</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">TargetDisk</span> <span class="n">sync</span><span class="o">=</span><span class="n">top</span> <span class="n">mode</span><span class="o">=</span><span class="n">existing</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;drive-mirror&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;mode&quot;</span><span class="p">:</span> <span class="s2">&quot;existing&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;target&quot;</span><span class="p">:</span> <span class="s2">&quot;nbd:localhost:49153:exportname=node-TargetDisk&quot;</span><span class="p">,</span>
        <span class="s2">&quot;sync&quot;</span><span class="p">:</span> <span class="s2">&quot;top&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">[On <em>source</em> QEMU] Once <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> copies the entire data, and the
event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code> is emitted, issue <code class="docutils literal notranslate"><span class="pre">block-job-cancel</span></code> to
gracefully end the synchronization, from source QEMU:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">job</span><span class="o">-</span><span class="n">cancel</span> <span class="n">device</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-job-cancel&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">[On <em>destination</em> QEMU] Then, stop the NBD server:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">nbd</span><span class="o">-</span><span class="n">server</span><span class="o">-</span><span class="n">stop</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;nbd-server-stop&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">[On <em>destination</em> QEMU] Finally, resume the guest vCPUs by issuing the
QMP command <cite>cont</cite>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">cont</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;cont&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
</ol>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Higher-level libraries (e.g. libvirt) automate the entire above
process (although note that libvirt does not allow same-host
migrations to localhost for other reasons).</p>
</div>
</div>
<div class="section" id="notes-on-blockdev-mirror">
<h3><a class="toc-backref" href="#id15">Notes on <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a><a class="headerlink" href="#notes-on-blockdev-mirror" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code> command is equivalent in core functionality to
<code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code>, except that it operates at node-level in a BDS graph.</p>
<p>Also: for <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code>, the ‘target’ image needs to be explicitly
created (using <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code>) and attach it to live QEMU via
<code class="docutils literal notranslate"><span class="pre">blockdev-add</span></code>, which assigns a name to the to-be created target node.</p>
<p>E.g. the sequence of actions to create a point-in-time backup of an
entire disk image chain, to a target, using <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code> would be:</p>
<ol class="arabic simple" start="0">
<li>Create the QCOW2 overlays, to arrive at a backing chain of desired
depth</li>
<li>Create the target image (using <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code>), say, <code class="docutils literal notranslate"><span class="pre">e.qcow2</span></code></li>
<li>Attach the above created file (<code class="docutils literal notranslate"><span class="pre">e.qcow2</span></code>), run-time, using
<code class="docutils literal notranslate"><span class="pre">blockdev-add</span></code> to QEMU</li>
<li>Perform <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code> (use <code class="docutils literal notranslate"><span class="pre">&quot;sync&quot;:</span> <span class="pre">&quot;full&quot;</span></code> to copy the
entire chain to the target).  And notice the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code></li>
<li>Optionally, query for active block jobs, there should be a ‘mirror’
job ready to be completed</li>
<li>Gracefully complete the ‘mirror’ block device job, and notice the
the event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code></li>
<li>Shutdown the guest by issuing the QMP <code class="docutils literal notranslate"><span class="pre">quit</span></code> command so that
caches are flushed</li>
<li>Then, finally, compare the contents of the disk image chain, and
the target copy with <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">compare</span></code>.  You should notice:
“Images are identical”</li>
</ol>
</div>
<div class="section" id="qmp-invocation-for-blockdev-mirror">
<h3><a class="toc-backref" href="#id16">QMP invocation for <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a><a class="headerlink" href="#qmp-invocation-for-blockdev-mirror" title="Permalink to this headline">¶</a></h3>
<p>Given the disk image chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>To copy the contents of the entire disk image chain, from [A] all the
way to [D], to a new target, call it [E].  The following is the flow.</p>
<p>Create the overlay images, [B], [C], and [D]:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">A</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">b</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">c</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">C</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
<span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">C</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">d</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
</pre></div>
</div>
<p>Create the target image, [E]:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-img create -f qcow2 e.qcow2 39M
</pre></div>
</div>
<p>Add the above created target image to QEMU, via <code class="docutils literal notranslate"><span class="pre">blockdev-add</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">add</span> <span class="n">driver</span><span class="o">=</span><span class="n">qcow2</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">E</span> <span class="n">file</span><span class="o">=</span><span class="p">{</span><span class="s2">&quot;driver&quot;</span><span class="p">:</span><span class="s2">&quot;file&quot;</span><span class="p">,</span><span class="s2">&quot;filename&quot;</span><span class="p">:</span><span class="s2">&quot;e.qcow2&quot;</span><span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;blockdev-add&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;node-name&quot;</span><span class="p">:</span> <span class="s2">&quot;node-E&quot;</span><span class="p">,</span>
        <span class="s2">&quot;driver&quot;</span><span class="p">:</span> <span class="s2">&quot;qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;file&quot;</span><span class="p">:</span> <span class="p">{</span>
            <span class="s2">&quot;driver&quot;</span><span class="p">:</span> <span class="s2">&quot;file&quot;</span><span class="p">,</span>
            <span class="s2">&quot;filename&quot;</span><span class="p">:</span> <span class="s2">&quot;e.qcow2&quot;</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Perform <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code>, and notice the event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_READY</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">mirror</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="n">target</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">E</span> <span class="n">sync</span><span class="o">=</span><span class="n">full</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;blockdev-mirror&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;target&quot;</span><span class="p">:</span> <span class="s2">&quot;node-E&quot;</span><span class="p">,</span>
        <span class="s2">&quot;sync&quot;</span><span class="p">:</span> <span class="s2">&quot;full&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Query for active block jobs, there should be a ‘mirror’ job ready:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">block</span><span class="o">-</span><span class="n">jobs</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;query-block-jobs&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;return&quot;</span><span class="p">:</span> <span class="p">[</span>
        <span class="p">{</span>
            <span class="s2">&quot;busy&quot;</span><span class="p">:</span> <span class="n">false</span><span class="p">,</span>
            <span class="s2">&quot;type&quot;</span><span class="p">:</span> <span class="s2">&quot;mirror&quot;</span><span class="p">,</span>
            <span class="s2">&quot;len&quot;</span><span class="p">:</span> <span class="mi">21561344</span><span class="p">,</span>
            <span class="s2">&quot;paused&quot;</span><span class="p">:</span> <span class="n">false</span><span class="p">,</span>
            <span class="s2">&quot;ready&quot;</span><span class="p">:</span> <span class="n">true</span><span class="p">,</span>
            <span class="s2">&quot;io-status&quot;</span><span class="p">:</span> <span class="s2">&quot;ok&quot;</span><span class="p">,</span>
            <span class="s2">&quot;offset&quot;</span><span class="p">:</span> <span class="mi">21561344</span><span class="p">,</span>
            <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
            <span class="s2">&quot;speed&quot;</span><span class="p">:</span> <span class="mi">0</span>
        <span class="p">}</span>
    <span class="p">]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Gracefully complete the block device job operation, and notice the
event <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">block</span><span class="o">-</span><span class="n">job</span><span class="o">-</span><span class="n">complete</span> <span class="n">device</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;block-job-complete&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;return&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Shutdown the guest, by issuing the <code class="docutils literal notranslate"><span class="pre">quit</span></code> QMP command:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">quit</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;quit&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="live-disk-backup-drive-backup-and-blockdev-backup">
<h2><a class="toc-backref" href="#id17">Live disk backup — <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> and <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a><a class="headerlink" href="#live-disk-backup-drive-backup-and-blockdev-backup" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> (and its newer equivalent <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code>) allows
you to create a point-in-time snapshot.</p>
<p>In this case, the point-in-time is when you <em>start</em> the <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code>
(or its newer equivalent <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code>) command.</p>
<div class="section" id="qmp-invocation-for-drive-backup">
<h3><a class="toc-backref" href="#id18">QMP invocation for <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code></a><a class="headerlink" href="#qmp-invocation-for-drive-backup" title="Permalink to this headline">¶</a></h3>
<p>Yet again, starting afresh with our example disk image chain:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">C</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">D</span><span class="p">]</span>
</pre></div>
</div>
<p>To create a target image [E], with content populated from image [A] to
[D], from the above chain, the following is the syntax.  (If the target
image does not exist, <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> will create it):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">drive</span><span class="o">-</span><span class="n">backup</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">D</span> <span class="n">sync</span><span class="o">=</span><span class="n">full</span> <span class="n">target</span><span class="o">=</span><span class="n">e</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;drive-backup&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-D&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;sync&quot;</span><span class="p">:</span> <span class="s2">&quot;full&quot;</span><span class="p">,</span>
        <span class="s2">&quot;target&quot;</span><span class="p">:</span> <span class="s2">&quot;e.qcow2&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Once the above <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> has completed, a <code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code> event
will be issued, indicating the live block device job operation has
completed, and no further action is required.</p>
</div>
<div class="section" id="notes-on-blockdev-backup">
<h3><a class="toc-backref" href="#id19">Notes on <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a><a class="headerlink" href="#notes-on-blockdev-backup" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code> command is equivalent in functionality to
<code class="docutils literal notranslate"><span class="pre">drive-backup</span></code>, except that it operates at node-level in a Block Driver
State (BDS) graph.</p>
<p>E.g. the sequence of actions to create a point-in-time backup
of an entire disk image chain, to a target, using <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code>
would be:</p>
<ol class="arabic simple" start="0">
<li>Create the QCOW2 overlays, to arrive at a backing chain of desired
depth</li>
<li>Create the target image (using <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code>), say, <code class="docutils literal notranslate"><span class="pre">e.qcow2</span></code></li>
<li>Attach the above created file (<code class="docutils literal notranslate"><span class="pre">e.qcow2</span></code>), run-time, using
<code class="docutils literal notranslate"><span class="pre">blockdev-add</span></code> to QEMU</li>
<li>Perform <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code> (use <code class="docutils literal notranslate"><span class="pre">&quot;sync&quot;:</span> <span class="pre">&quot;full&quot;</span></code> to copy the
entire chain to the target).  And notice the event
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code></li>
<li>Shutdown the guest, by issuing the QMP <code class="docutils literal notranslate"><span class="pre">quit</span></code> command, so that
caches are flushed</li>
<li>Then, finally, compare the contents of the disk image chain, and
the target copy with <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">compare</span></code>.  You should notice:
“Images are identical”</li>
</ol>
<p>The following section shows an example QMP invocation for
<code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code>.</p>
</div>
<div class="section" id="qmp-invocation-for-blockdev-backup">
<h3><a class="toc-backref" href="#id20">QMP invocation for <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a><a class="headerlink" href="#qmp-invocation-for-blockdev-backup" title="Permalink to this headline">¶</a></h3>
<p>Given a disk image chain of depth 1 where image [B] is the active
overlay (live QEMU is writing to it):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">A</span><span class="p">]</span> <span class="o">&lt;--</span> <span class="p">[</span><span class="n">B</span><span class="p">]</span>
</pre></div>
</div>
<p>The following is the procedure to copy the content from the entire chain
to a target image (say, [E]), which has the full content from [A] and
[B].</p>
<p>Create the overlay [B]:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">snapshot</span><span class="o">-</span><span class="n">sync</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">A</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">file</span><span class="o">=</span><span class="n">b</span><span class="o">.</span><span class="n">qcow2</span> <span class="n">snapshot</span><span class="o">-</span><span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="nb">format</span><span class="o">=</span><span class="n">qcow2</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;blockdev-snapshot-sync&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;node-name&quot;</span><span class="p">:</span> <span class="s2">&quot;node-A&quot;</span><span class="p">,</span>
        <span class="s2">&quot;snapshot-file&quot;</span><span class="p">:</span> <span class="s2">&quot;b.qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;format&quot;</span><span class="p">:</span> <span class="s2">&quot;qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;snapshot-node-name&quot;</span><span class="p">:</span> <span class="s2">&quot;node-B&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Create a target image that will contain the copy:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-img create -f qcow2 e.qcow2 39M
</pre></div>
</div>
<p>Then add it to QEMU via <code class="docutils literal notranslate"><span class="pre">blockdev-add</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">add</span> <span class="n">driver</span><span class="o">=</span><span class="n">qcow2</span> <span class="n">node</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">E</span> <span class="n">file</span><span class="o">=</span><span class="p">{</span><span class="s2">&quot;driver&quot;</span><span class="p">:</span><span class="s2">&quot;file&quot;</span><span class="p">,</span><span class="s2">&quot;filename&quot;</span><span class="p">:</span><span class="s2">&quot;e.qcow2&quot;</span><span class="p">}</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;blockdev-add&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;node-name&quot;</span><span class="p">:</span> <span class="s2">&quot;node-E&quot;</span><span class="p">,</span>
        <span class="s2">&quot;driver&quot;</span><span class="p">:</span> <span class="s2">&quot;qcow2&quot;</span><span class="p">,</span>
        <span class="s2">&quot;file&quot;</span><span class="p">:</span> <span class="p">{</span>
            <span class="s2">&quot;driver&quot;</span><span class="p">:</span> <span class="s2">&quot;file&quot;</span><span class="p">,</span>
            <span class="s2">&quot;filename&quot;</span><span class="p">:</span> <span class="s2">&quot;e.qcow2&quot;</span>
        <span class="p">}</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Then invoke <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code> to copy the contents from the entire
image chain, consisting of images [A] and [B] to the target image
‘e.qcow2’:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">blockdev</span><span class="o">-</span><span class="n">backup</span> <span class="n">device</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">B</span> <span class="n">target</span><span class="o">=</span><span class="n">node</span><span class="o">-</span><span class="n">E</span> <span class="n">sync</span><span class="o">=</span><span class="n">full</span> <span class="n">job</span><span class="o">-</span><span class="nb">id</span><span class="o">=</span><span class="n">job0</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;blockdev-backup&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{</span>
        <span class="s2">&quot;device&quot;</span><span class="p">:</span> <span class="s2">&quot;node-B&quot;</span><span class="p">,</span>
        <span class="s2">&quot;job-id&quot;</span><span class="p">:</span> <span class="s2">&quot;job0&quot;</span><span class="p">,</span>
        <span class="s2">&quot;target&quot;</span><span class="p">:</span> <span class="s2">&quot;node-E&quot;</span><span class="p">,</span>
        <span class="s2">&quot;sync&quot;</span><span class="p">:</span> <span class="s2">&quot;full&quot;</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Once the above ‘backup’ operation has completed, the event,
<code class="docutils literal notranslate"><span class="pre">BLOCK_JOB_COMPLETED</span></code> will be emitted, signalling successful
completion.</p>
<p>Next, query for any active block device jobs (there should be none):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">query</span><span class="o">-</span><span class="n">block</span><span class="o">-</span><span class="n">jobs</span>
<span class="p">{</span>
    <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;query-block-jobs&quot;</span><span class="p">,</span>
    <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Shutdown the guest:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">QEMU</span><span class="p">)</span> <span class="n">quit</span>
<span class="p">{</span>
        <span class="s2">&quot;execute&quot;</span><span class="p">:</span> <span class="s2">&quot;quit&quot;</span><span class="p">,</span>
            <span class="s2">&quot;arguments&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
        <span class="s2">&quot;return&quot;</span><span class="p">:</span> <span class="p">{}</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The above step is really important; if forgotten, an error, “Failed
to get shared “write” lock on e.qcow2”, will be thrown when you do
<code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">compare</span></code> to verify the integrity of the disk image
with the backup content.</p>
</div>
<p>The end result will be the image ‘e.qcow2’ containing a
point-in-time backup of the disk image chain – i.e. contents from
images [A] and [B] at the time the <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code> command was
initiated.</p>
<p>One way to confirm the backup disk image contains the identical content
with the disk image chain is to compare the backup and the contents of
the chain, you should see “Images are identical”.  (NB: this is assuming
QEMU was launched with <code class="docutils literal notranslate"><span class="pre">-S</span></code> option, which will not start the CPUs at
guest boot up):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-img compare b.qcow2 e.qcow2
Warning: Image size mismatch!
Images are identical.
</pre></div>
</div>
<p>NOTE: The “Warning: Image size mismatch!” is expected, as we created the
target image (e.qcow2) with 39M size.</p>
</div>
</div>
</div>


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<h1 class="logo"><a href="index.html">QEMU</a></h1>



<p class="blurb">System Emulation Management and Interoperability Guide</p>






<h3>Navigation</h3>
<ul class="current">
<li class="toctree-l1"><a class="reference internal" href="bitmaps.html">Dirty Bitmaps and Incremental Backup</a></li>
<li class="toctree-l1 current"><a class="current reference internal" href="#">Live Block Device Operations</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#disk-image-backing-chain-notation">Disk image backing chain notation</a></li>
<li class="toctree-l2"><a class="reference internal" href="#brief-overview-of-live-block-qmp-primitives">Brief overview of live block QMP primitives</a></li>
<li class="toctree-l2"><a class="reference internal" href="#interacting-with-a-qemu-instance">Interacting with a QEMU instance</a></li>
<li class="toctree-l2"><a class="reference internal" href="#example-disk-image-chain">Example disk image chain</a></li>
<li class="toctree-l2"><a class="reference internal" href="#a-note-on-points-in-time-vs-file-names">A note on points-in-time vs file names</a></li>
<li class="toctree-l2"><a class="reference internal" href="#live-block-streaming-block-stream">Live block streaming — <code class="docutils literal notranslate"><span class="pre">block-stream</span></code></a></li>
<li class="toctree-l2"><a class="reference internal" href="#live-block-commit-block-commit">Live block commit — <code class="docutils literal notranslate"><span class="pre">block-commit</span></code></a></li>
<li class="toctree-l2"><a class="reference internal" href="#live-disk-synchronization-drive-mirror-and-blockdev-mirror">Live disk synchronization — <code class="docutils literal notranslate"><span class="pre">drive-mirror</span></code> and <code class="docutils literal notranslate"><span class="pre">blockdev-mirror</span></code></a></li>
<li class="toctree-l2"><a class="reference internal" href="#live-disk-backup-drive-backup-and-blockdev-backup">Live disk backup — <code class="docutils literal notranslate"><span class="pre">drive-backup</span></code> and <code class="docutils literal notranslate"><span class="pre">blockdev-backup</span></code></a></li>
</ul>
</li>
<li class="toctree-l1"><a class="reference internal" href="pr-helper.html">Persistent reservation helper protocol</a></li>
<li class="toctree-l1"><a class="reference internal" href="qemu-ga.html">QEMU Guest Agent</a></li>
<li class="toctree-l1"><a class="reference internal" href="vhost-user.html">Vhost-user Protocol</a></li>
<li class="toctree-l1"><a class="reference internal" href="vhost-user-gpu.html">Vhost-user-gpu Protocol</a></li>
</ul>


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