diff options
author | Stefan Hajnoczi <stefanha@redhat.com> | 2019-08-29 14:41:04 +0100 |
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committer | Miklos Szeredi <mszeredi@redhat.com> | 2019-09-18 15:09:34 +0200 |
commit | 2d1d25d0a224dcd2021004d52342fc1727ccd85f (patch) | |
tree | a84c8df36800dc29b5353f4df660ad5ed1eaf772 /Documentation/filesystems | |
parent | c4bb667eaf520f21b3a3db0489682becc9c49bcc (diff) |
virtio-fs: add Documentation/filesystems/virtiofs.rst
Add information about the new "virtiofs" file system.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r-- | Documentation/filesystems/index.rst | 10 | ||||
-rw-r--r-- | Documentation/filesystems/virtiofs.rst | 60 |
2 files changed, 70 insertions, 0 deletions
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst index 2de2fe2ab078..56e94bfc580f 100644 --- a/Documentation/filesystems/index.rst +++ b/Documentation/filesystems/index.rst @@ -32,3 +32,13 @@ filesystem implementations. journalling fscrypt + +Filesystems +=========== + +Documentation for filesystem implementations. + +.. toctree:: + :maxdepth: 2 + + virtiofs diff --git a/Documentation/filesystems/virtiofs.rst b/Documentation/filesystems/virtiofs.rst new file mode 100644 index 000000000000..4f338e3cb3f7 --- /dev/null +++ b/Documentation/filesystems/virtiofs.rst @@ -0,0 +1,60 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=================================================== +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. + +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. |