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<li class="toctree-l3"><a class="reference internal" href="#defining-a-persistent-reservation-manager">Defining a persistent reservation manager</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#multipath-devices-and-persistent-reservations">Multipath devices and persistent reservations</a></li>
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<section id="persistent-reservation-managers">
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<h1>Persistent reservation managers<a class="headerlink" href="#persistent-reservation-managers" title="Link to this heading"></a></h1>
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<p>SCSI persistent reservations allow restricting access to block devices
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to specific initiators in a shared storage setup. When implementing
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clustering of virtual machines, it is a common requirement for virtual
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machines to send persistent reservation SCSI commands. However,
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the operating system restricts sending these commands to unprivileged
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programs because incorrect usage can disrupt regular operation of the
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storage fabric.</p>
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<p>For this reason, QEMU’s SCSI passthrough devices, <code class="docutils literal notranslate"><span class="pre">scsi-block</span></code>
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and <code class="docutils literal notranslate"><span class="pre">scsi-generic</span></code> (both are only available on Linux) can delegate
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implementation of persistent reservations to a separate object,
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the “persistent reservation manager”. Only PERSISTENT RESERVE OUT and
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PERSISTENT RESERVE IN commands are passed to the persistent reservation
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manager object; other commands are processed by QEMU as usual.</p>
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<section id="defining-a-persistent-reservation-manager">
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<h2>Defining a persistent reservation manager<a class="headerlink" href="#defining-a-persistent-reservation-manager" title="Link to this heading"></a></h2>
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<p>A persistent reservation manager is an instance of a subclass of the
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“pr-manager” QOM class.</p>
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<p>Right now only one subclass is defined, <code class="docutils literal notranslate"><span class="pre">pr-manager-helper</span></code>, which
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forwards the commands to an external privileged helper program
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over Unix sockets. The helper program only allows sending persistent
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reservation commands to devices for which QEMU has a file descriptor,
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so that QEMU will not be able to effect persistent reservations
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unless it has access to both the socket and the device.</p>
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<p><code class="docutils literal notranslate"><span class="pre">pr-manager-helper</span></code> has a single string property, <code class="docutils literal notranslate"><span class="pre">path</span></code>, which
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accepts the path to the helper program’s Unix socket. For example,
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the following command line defines a <code class="docutils literal notranslate"><span class="pre">pr-manager-helper</span></code> object and
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attaches it to a SCSI passthrough device:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64
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-device virtio-scsi \
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-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
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-drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
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-device scsi-block,drive=hd
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</pre></div>
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</div>
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<p>Alternatively, using <code class="docutils literal notranslate"><span class="pre">-blockdev</span></code>:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64
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-device virtio-scsi \
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-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
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-blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
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-device scsi-block,drive=hd
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</pre></div>
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</div>
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<p>You will also need to ensure that the helper program
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<strong class="command">qemu-pr-helper</strong> is running, and that it has been
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set up to use the same socket filename as your QEMU commandline
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specifies. See the qemu-pr-helper documentation or manpage for
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further details.</p>
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</section>
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<section id="multipath-devices-and-persistent-reservations">
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<h2>Multipath devices and persistent reservations<a class="headerlink" href="#multipath-devices-and-persistent-reservations" title="Link to this heading"></a></h2>
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<p>Proper support of persistent reservation for multipath devices requires
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communication with the multipath daemon, so that the reservation is
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registered and applied when a path is newly discovered or becomes online
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again. <strong class="command">qemu-pr-helper</strong> can do this if the <code class="docutils literal notranslate"><span class="pre">libmpathpersist</span></code>
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library was available on the system at build time.</p>
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<p>As of August 2017, a reservation key must be specified in <code class="docutils literal notranslate"><span class="pre">multipath.conf</span></code>
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for <code class="docutils literal notranslate"><span class="pre">multipathd</span></code> to check for persistent reservation for newly
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discovered paths or reinstated paths. The attribute can be added
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to the <code class="docutils literal notranslate"><span class="pre">defaults</span></code> section or the <code class="docutils literal notranslate"><span class="pre">multipaths</span></code> section; for example:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">multipaths</span> <span class="p">{</span>
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<span class="n">multipath</span> <span class="p">{</span>
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<span class="n">wwid</span> <span class="n">XXXXXXXXXXXXXXXX</span>
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<span class="n">alias</span> <span class="n">yellow</span>
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<span class="n">reservation_key</span> <span class="mh">0x123abc</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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</div>
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<p>Linking <strong class="program">qemu-pr-helper</strong> to <code class="docutils literal notranslate"><span class="pre">libmpathpersist</span></code> does not impede
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its usage on regular SCSI devices.</p>
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