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<section id="virtio-pmem">
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<h1>virtio pmem<a class="headerlink" href="#virtio-pmem" title="Link to this heading"></a></h1>
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<p>This document explains the setup and usage of the virtio pmem device.
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The virtio pmem device is a paravirtualized persistent memory device
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on regular (i.e non-NVDIMM) storage.</p>
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<section id="usecase">
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<h2>Usecase<a class="headerlink" href="#usecase" title="Link to this heading"></a></h2>
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<p>Virtio pmem allows to bypass the guest page cache and directly use
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host page cache. This reduces guest memory footprint as the host can
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make efficient memory reclaim decisions under memory pressure.</p>
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</section>
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<section id="how-does-virtio-pmem-compare-to-the-nvdimm-emulation">
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<h2>How does virtio-pmem compare to the nvdimm emulation?<a class="headerlink" href="#how-does-virtio-pmem-compare-to-the-nvdimm-emulation" title="Link to this heading"></a></h2>
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<p>NVDIMM emulation on regular (i.e. non-NVDIMM) host storage does not
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persist the guest writes as there are no defined semantics in the device
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specification. The virtio pmem device provides guest write persistence
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on non-NVDIMM host storage.</p>
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</section>
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<section id="virtio-pmem-usage">
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<h2>virtio pmem usage<a class="headerlink" href="#virtio-pmem-usage" title="Link to this heading"></a></h2>
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<p>A virtio pmem device backed by a memory-backend-file can be created on
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the QEMU command line as in the following example:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="nb">object</span> <span class="n">memory</span><span class="o">-</span><span class="n">backend</span><span class="o">-</span><span class="n">file</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">mem1</span><span class="p">,</span><span class="n">share</span><span class="p">,</span><span class="n">mem</span><span class="o">-</span><span class="n">path</span><span class="o">=./</span><span class="n">virtio_pmem</span><span class="o">.</span><span class="n">img</span><span class="p">,</span><span class="n">size</span><span class="o">=</span><span class="mi">4</span><span class="n">G</span>
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<span class="o">-</span><span class="n">device</span> <span class="n">virtio</span><span class="o">-</span><span class="n">pmem</span><span class="o">-</span><span class="n">pci</span><span class="p">,</span><span class="n">memdev</span><span class="o">=</span><span class="n">mem1</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">nv1</span>
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</pre></div>
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</div>
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<p>where:</p>
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<blockquote>
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<div><ul class="simple">
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<li><p>“object memory-backend-file,id=mem1,share,mem-path=<image>, size=<image size>”
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creates a backend file with the specified size.</p></li>
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<li><p>“device virtio-pmem-pci,id=nvdimm1,memdev=mem1” creates a virtio pmem
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pci device whose storage is provided by above memory backend device.</p></li>
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</ul>
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</div></blockquote>
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<p>Multiple virtio pmem devices can be created if multiple pairs of “-object”
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and “-device” are provided.</p>
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</section>
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<section id="hotplug">
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<h2>Hotplug<a class="headerlink" href="#hotplug" title="Link to this heading"></a></h2>
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<p>Virtio pmem devices can be hotplugged via the QEMU monitor. First, the
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memory backing has to be added via ‘object_add’; afterwards, the virtio
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pmem device can be added via ‘device_add’.</p>
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<p>For example, the following commands add another 4GB virtio pmem device to
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the guest:</p>
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<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">object_add</span> <span class="n">memory</span><span class="o">-</span><span class="n">backend</span><span class="o">-</span><span class="n">file</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">mem2</span><span class="p">,</span><span class="n">share</span><span class="o">=</span><span class="n">on</span><span class="p">,</span><span class="n">mem</span><span class="o">-</span><span class="n">path</span><span class="o">=</span><span class="n">virtio_pmem2</span><span class="o">.</span><span class="n">img</span><span class="p">,</span><span class="n">size</span><span class="o">=</span><span class="mi">4</span><span class="n">G</span>
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<span class="p">(</span><span class="n">qemu</span><span class="p">)</span> <span class="n">device_add</span> <span class="n">virtio</span><span class="o">-</span><span class="n">pmem</span><span class="o">-</span><span class="n">pci</span><span class="p">,</span><span class="nb">id</span><span class="o">=</span><span class="n">virtio_pmem2</span><span class="p">,</span><span class="n">memdev</span><span class="o">=</span><span class="n">mem2</span>
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</pre></div>
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</div>
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</section>
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<section id="guest-data-persistence">
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<h2>Guest Data Persistence<a class="headerlink" href="#guest-data-persistence" title="Link to this heading"></a></h2>
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<p>Guest data persistence on non-NVDIMM requires guest userspace applications
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to perform fsync/msync. This is different from a real nvdimm backend where
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no additional fsync/msync is required. This is to persist guest writes in
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host backing file which otherwise remains in host page cache and there is
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risk of losing the data in case of power failure.</p>
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<p>With virtio pmem device, MAP_SYNC mmap flag is not supported. This provides
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a hint to application to perform fsync for write persistence.</p>
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</section>
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<section id="limitations">
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<h2>Limitations<a class="headerlink" href="#limitations" title="Link to this heading"></a></h2>
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<ul class="simple">
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<li><p>Real nvdimm device backend is not supported.</p></li>
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<li><p>virtio pmem hotunplug is not supported.</p></li>
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<li><p>ACPI NVDIMM features like regions/namespaces are not supported.</p></li>
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<li><p>ndctl command is not supported.</p></li>
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</ul>
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