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<!DOCTYPE html> <!--[if IE 8]><html class="no-js lt-ie9" lang="en" > <![endif]--> <!--[if gt IE 8]><!--> <html class="no-js" lang="en" > <!--<![endif]--> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>Persistent reservation managers — QEMU qemu-kvm-6.2.0-53.module+el8.10.0+2055+8eb7870b.4 documentation</title> <link rel="shortcut icon" href="../_static/qemu_32x32.png"/> <link rel="stylesheet" href="../_static/css/theme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="next" title="QEMU System Emulator Targets" href="targets.html" /> <link rel="prev" title="Virtual CPU hotplug" href="cpu-hotplug.html" /> <script src="../_static/js/modernizr.min.js"></script> </head> <body class="wy-body-for-nav"> <div class="wy-grid-for-nav"> <nav 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properties</a></li> <li class="toctree-l2"><a class="reference internal" href="cpu-hotplug.html">Virtual CPU hotplug</a></li> <li class="toctree-l2 current"><a class="current reference internal" href="#">Persistent reservation managers</a><ul> <li class="toctree-l3"><a class="reference internal" href="#defining-a-persistent-reservation-manager">Defining a persistent reservation manager</a></li> <li class="toctree-l3"><a class="reference internal" href="#multipath-devices-and-persistent-reservations">Multipath devices and persistent reservations</a></li> </ul> </li> <li class="toctree-l2"><a class="reference internal" href="targets.html">QEMU System Emulator Targets</a></li> <li class="toctree-l2"><a class="reference internal" href="security.html">Security</a></li> <li class="toctree-l2"><a class="reference internal" href="multi-process.html">Multi-process QEMU</a></li> </ul> </li> <li class="toctree-l1"><a class="reference internal" href="../user/index.html">User Mode Emulation</a></li> <li class="toctree-l1"><a class="reference internal" href="../tools/index.html">Tools</a></li> <li class="toctree-l1"><a class="reference internal" href="../interop/index.html">System Emulation Management and Interoperability</a></li> <li class="toctree-l1"><a class="reference internal" href="../specs/index.html">System Emulation Guest Hardware Specifications</a></li> <li class="toctree-l1"><a class="reference internal" href="../devel/index.html">Developer Information</a></li> </ul> </div> </div> </nav> <section data-toggle="wy-nav-shift" class="wy-nav-content-wrap"> <nav class="wy-nav-top" aria-label="top navigation"> <i data-toggle="wy-nav-top" class="fa fa-bars"></i> <a href="../index.html">QEMU</a> </nav> <div class="wy-nav-content"> <div class="rst-content"> <div role="navigation" aria-label="breadcrumbs navigation"> <ul class="wy-breadcrumbs"> <li><a href="../index.html">Docs</a> »</li> <li><a href="index.html">System Emulation</a> »</li> <li>Persistent reservation managers</li> <li class="wy-breadcrumbs-aside"> <a href="https://gitlab.com/qemu-project/qemu/blob/master/docs/system/pr-manager.rst" class="fa fa-gitlab"> Edit on GitLab</a> </li> </ul> <hr/> </div> <div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article"> <div itemprop="articleBody"> <div class="section" id="persistent-reservation-managers"> <h1>Persistent reservation managers<a class="headerlink" href="#persistent-reservation-managers" title="Permalink to this headline">¶</a></h1> <p>SCSI persistent reservations allow restricting access to block devices to specific initiators in a shared storage setup. When implementing clustering of virtual machines, it is a common requirement for virtual machines to send persistent reservation SCSI commands. However, the operating system restricts sending these commands to unprivileged programs because incorrect usage can disrupt regular operation of the storage fabric.</p> <p>For this reason, QEMU’s SCSI passthrough devices, <code class="docutils literal notranslate"><span class="pre">scsi-block</span></code> and <code class="docutils literal notranslate"><span class="pre">scsi-generic</span></code> (both are only available on Linux) can delegate implementation of persistent reservations to a separate object, the “persistent reservation manager”. Only PERSISTENT RESERVE OUT and PERSISTENT RESERVE IN commands are passed to the persistent reservation manager object; other commands are processed by QEMU as usual.</p> <div class="section" id="defining-a-persistent-reservation-manager"> <h2>Defining a persistent reservation manager<a class="headerlink" href="#defining-a-persistent-reservation-manager" title="Permalink to this headline">¶</a></h2> <p>A persistent reservation manager is an instance of a subclass of the “pr-manager” QOM class.</p> <p>Right now only one subclass is defined, <code class="docutils literal notranslate"><span class="pre">pr-manager-helper</span></code>, which forwards the commands to an external privileged helper program over Unix sockets. The helper program only allows sending persistent reservation commands to devices for which QEMU has a file descriptor, so that QEMU will not be able to effect persistent reservations unless it has access to both the socket and the device.</p> <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 accepts the path to the helper program’s Unix socket. For example, the following command line defines a <code class="docutils literal notranslate"><span class="pre">pr-manager-helper</span></code> object and attaches it to a SCSI passthrough device:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64 -device virtio-scsi \ -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0 -device scsi-block,drive=hd </pre></div> </div> <p>Alternatively, using <code class="docutils literal notranslate"><span class="pre">-blockdev</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ qemu-system-x86_64 -device virtio-scsi \ -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0 -device scsi-block,drive=hd </pre></div> </div> <p>You will also need to ensure that the helper program <strong class="command">qemu-pr-helper</strong> is running, and that it has been set up to use the same socket filename as your QEMU commandline specifies. See the qemu-pr-helper documentation or manpage for further details.</p> </div> <div class="section" id="multipath-devices-and-persistent-reservations"> <h2>Multipath devices and persistent reservations<a class="headerlink" href="#multipath-devices-and-persistent-reservations" title="Permalink to this headline">¶</a></h2> <p>Proper support of persistent reservation for multipath devices requires communication with the multipath daemon, so that the reservation is registered and applied when a path is newly discovered or becomes online again. <strong class="command">qemu-pr-helper</strong> can do this if the <code class="docutils literal notranslate"><span class="pre">libmpathpersist</span></code> library was available on the system at build time.</p> <p>As of August 2017, a reservation key must be specified in <code class="docutils literal notranslate"><span class="pre">multipath.conf</span></code> for <code class="docutils literal notranslate"><span class="pre">multipathd</span></code> to check for persistent reservation for newly discovered paths or reinstated paths. The attribute can be added 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> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">multipaths</span> <span class="p">{</span> <span class="n">multipath</span> <span class="p">{</span> <span class="n">wwid</span> <span class="n">XXXXXXXXXXXXXXXX</span> <span class="n">alias</span> <span class="n">yellow</span> <span class="n">reservation_key</span> <span class="mh">0x123abc</span> <span class="p">}</span> <span class="p">}</span> </pre></div> </div> <p>Linking <strong class="program">qemu-pr-helper</strong> to <code class="docutils literal notranslate"><span class="pre">libmpathpersist</span></code> does not impede its usage on regular SCSI devices.</p> </div> </div> </div> </div> <footer> <div class="rst-footer-buttons" role="navigation" aria-label="footer navigation"> <a href="targets.html" class="btn btn-neutral float-right" title="QEMU System Emulator Targets" accesskey="n" rel="next">Next <span class="fa fa-arrow-circle-right"></span></a> <a href="cpu-hotplug.html" class="btn btn-neutral" title="Virtual CPU hotplug" accesskey="p" rel="prev"><span class="fa fa-arrow-circle-left"></span> Previous</a> </div> <hr/> <div role="contentinfo"> <p> © Copyright 2021, The QEMU Project Developers. </p> </div> Built with <a href="http://sphinx-doc.org/">Sphinx</a> using a <a href="https://github.com/rtfd/sphinx_rtd_theme">theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a>. <!-- Empty para to force a blank line after "Built with Sphinx ..." --> <p></p> <p>This documentation is for QEMU version 6.2.0.</p> <p><a href="../about/license.html">QEMU and this manual are released under the GNU General Public License, version 2.</a></p> </footer> </div> </div> </section> </div> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT:'../', VERSION:'qemu-kvm-6.2.0-53.module+el8.10.0+2055+8eb7870b.4', LANGUAGE:'None', COLLAPSE_INDEX:false, FILE_SUFFIX:'.html', HAS_SOURCE: false, SOURCELINK_SUFFIX: '.txt' }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/js/theme.js"></script> <script type="text/javascript"> jQuery(function () { SphinxRtdTheme.Navigation.enable(true); }); </script> </body> </html>