77 lines
3.0 KiB
ReStructuredText
77 lines
3.0 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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.. _virtiofs_index:
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===================================================
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virtiofs: virtio-fs host<->guest shared file system
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===================================================
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- Copyright (C) 2019 Red Hat, Inc.
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Introduction
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============
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The virtiofs file system for Linux implements a driver for the paravirtualized
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VIRTIO "virtio-fs" device for guest<->host file system sharing. It allows a
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guest to mount a directory that has been exported on the host.
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Guests often require access to files residing on the host or remote systems.
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Use cases include making files available to new guests during installation,
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booting from a root file system located on the host, persistent storage for
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stateless or ephemeral guests, and sharing a directory between guests.
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Although it is possible to use existing network file systems for some of these
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tasks, they require configuration steps that are hard to automate and they
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expose the storage network to the guest. The virtio-fs device was designed to
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solve these problems by providing file system access without networking.
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Furthermore the virtio-fs device takes advantage of the co-location of the
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guest and host to increase performance and provide semantics that are not
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possible with network file systems.
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Usage
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=====
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Mount file system with tag ``myfs`` on ``/mnt``:
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.. code-block:: sh
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guest# mount -t virtiofs myfs /mnt
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Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU
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and the virtiofsd daemon.
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Mount options
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-------------
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virtiofs supports general VFS mount options, for example, remount,
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ro, rw, context, etc. It also supports FUSE mount options.
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atime behavior
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^^^^^^^^^^^^^^
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The atime-related mount options, for example, noatime, strictatime,
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are ignored. The atime behavior for virtiofs is the same as the
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underlying filesystem of the directory that has been exported
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on the host.
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Internals
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=========
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Since the virtio-fs device uses the FUSE protocol for file system requests, the
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virtiofs file system for Linux is integrated closely with the FUSE file system
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client. The guest acts as the FUSE client while the host acts as the FUSE
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server. The /dev/fuse interface between the kernel and userspace is replaced
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with the virtio-fs device interface.
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FUSE requests are placed into a virtqueue and processed by the host. The
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response portion of the buffer is filled in by the host and the guest handles
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the request completion.
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Mapping /dev/fuse to virtqueues requires solving differences in semantics
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between /dev/fuse and virtqueues. Each time the /dev/fuse device is read, the
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FUSE client may choose which request to transfer, making it possible to
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prioritize certain requests over others. Virtqueues have queue semantics and
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it is not possible to change the order of requests that have been enqueued.
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This is especially important if the virtqueue becomes full since it is then
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impossible to add high priority requests. In order to address this difference,
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the virtio-fs device uses a "hiprio" virtqueue specifically for requests that
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have priority over normal requests.
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