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<section id="virtiofs-virtio-fs-host-guest-shared-file-system">
<span id="virtiofs-index"></span><h1>virtiofs: virtio-fs host<->guest shared file system<a class="headerlink" href="#virtiofs-virtio-fs-host-guest-shared-file-system" title="Link to this heading">¶</a></h1>
<ul class="simple">
<li><p>Copyright (C) 2019 Red Hat, Inc.</p></li>
</ul>
<section id="introduction">
<h2>Introduction<a class="headerlink" href="#introduction" title="Link to this heading">¶</a></h2>
<p>The virtiofs file system for Linux implements a driver for the paravirtualized
VIRTIO “virtio-fs” device for guest<->host file system sharing. It allows a
guest to mount a directory that has been exported on the host.</p>
<p>Guests often require access to files residing on the host or remote systems.
Use cases include making files available to new guests during installation,
booting from a root file system located on the host, persistent storage for
stateless or ephemeral guests, and sharing a directory between guests.</p>
<p>Although it is possible to use existing network file systems for some of these
tasks, they require configuration steps that are hard to automate and they
expose the storage network to the guest. The virtio-fs device was designed to
solve these problems by providing file system access without networking.</p>
<p>Furthermore the virtio-fs device takes advantage of the co-location of the
guest and host to increase performance and provide semantics that are not
possible with network file systems.</p>
</section>
<section id="usage">
<h2>Usage<a class="headerlink" href="#usage" title="Link to this heading">¶</a></h2>
<p>Mount file system with tag <code class="docutils literal notranslate"><span class="pre">myfs</span></code> on <code class="docutils literal notranslate"><span class="pre">/mnt</span></code>:</p>
<div class="highlight-sh notranslate"><div class="highlight"><pre><span></span>guest#<span class="w"> </span>mount<span class="w"> </span>-t<span class="w"> </span>virtiofs<span class="w"> </span>myfs<span class="w"> </span>/mnt
</pre></div>
</div>
<p>Please see <a class="reference external" href="https://virtio-fs.gitlab.io/">https://virtio-fs.gitlab.io/</a> for details on how to configure QEMU
and the virtiofsd daemon.</p>
<section id="mount-options">
<h3>Mount options<a class="headerlink" href="#mount-options" title="Link to this heading">¶</a></h3>
<p>virtiofs supports general VFS mount options, for example, remount,
ro, rw, context, etc. It also supports FUSE mount options.</p>
<section id="atime-behavior">
<h4>atime behavior<a class="headerlink" href="#atime-behavior" title="Link to this heading">¶</a></h4>
<p>The atime-related mount options, for example, noatime, strictatime,
are ignored. The atime behavior for virtiofs is the same as the
underlying filesystem of the directory that has been exported
on the host.</p>
</section>
</section>
</section>
<section id="internals">
<h2>Internals<a class="headerlink" href="#internals" title="Link to this heading">¶</a></h2>
<p>Since the virtio-fs device uses the FUSE protocol for file system requests, the
virtiofs file system for Linux is integrated closely with the FUSE file system
client. The guest acts as the FUSE client while the host acts as the FUSE
server. The /dev/fuse interface between the kernel and userspace is replaced
with the virtio-fs device interface.</p>
<p>FUSE requests are placed into a virtqueue and processed by the host. The
response portion of the buffer is filled in by the host and the guest handles
the request completion.</p>
<p>Mapping /dev/fuse to virtqueues requires solving differences in semantics
between /dev/fuse and virtqueues. Each time the /dev/fuse device is read, the
FUSE client may choose which request to transfer, making it possible to
prioritize certain requests over others. Virtqueues have queue semantics and
it is not possible to change the order of requests that have been enqueued.
This is especially important if the virtqueue becomes full since it is then
impossible to add high priority requests. In order to address this difference,
the virtio-fs device uses a “hiprio” virtqueue specifically for requests that
have priority over normal requests.</p>
</section>
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