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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 10:05:51 +0000 |
commit | 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 (patch) | |
tree | a94efe259b9009378be6d90eb30d2b019d95c194 /Documentation/filesystems/virtiofs.rst | |
parent | Initial commit. (diff) | |
download | linux-430c2fc249ea5c0536abd21c23382884005c9093.tar.xz linux-430c2fc249ea5c0536abd21c23382884005c9093.zip |
Adding upstream version 5.10.209.upstream/5.10.209upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'Documentation/filesystems/virtiofs.rst')
-rw-r--r-- | Documentation/filesystems/virtiofs.rst | 76 |
1 files changed, 76 insertions, 0 deletions
diff --git a/Documentation/filesystems/virtiofs.rst b/Documentation/filesystems/virtiofs.rst new file mode 100644 index 000000000..fd4d2484e --- /dev/null +++ b/Documentation/filesystems/virtiofs.rst @@ -0,0 +1,76 @@ +.. SPDX-License-Identifier: GPL-2.0 + +.. _virtiofs_index: + +=================================================== +virtiofs: virtio-fs host<->guest shared file system +=================================================== + +- Copyright (C) 2019 Red Hat, Inc. + +Introduction +============ +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. + +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. + +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. + +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. + +Usage +===== +Mount file system with tag ``myfs`` on ``/mnt``: + +.. code-block:: sh + + guest# mount -t virtiofs myfs /mnt + +Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU +and the virtiofsd daemon. + +Mount options +------------- + +virtiofs supports general VFS mount options, for example, remount, +ro, rw, context, etc. It also supports FUSE mount options. + +atime behavior +^^^^^^^^^^^^^^ + +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. + +Internals +========= +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. + +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. + +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. |