projects / linux-2.6-block.git / blame
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1 | .. SPDX-License-Identifier: GPL-2.0 |
| 2 | ||
| 3 | =================================================== | |
| 4 | virtiofs: virtio-fs host<->guest shared file system | |
| 5 | =================================================== | |
| 6 | ||
| 7 | - Copyright (C) 2019 Red Hat, Inc. | |
| 8 | ||
| 9 | Introduction | |
| 10 | ============ | |
| 11 | The virtiofs file system for Linux implements a driver for the paravirtualized | |
| 12 | VIRTIO "virtio-fs" device for guest<->host file system sharing. It allows a | |
| 13 | guest to mount a directory that has been exported on the host. | |
| 14 | ||
| 15 | Guests often require access to files residing on the host or remote systems. | |
| 16 | Use cases include making files available to new guests during installation, | |
| 17 | booting from a root file system located on the host, persistent storage for | |
| 18 | stateless or ephemeral guests, and sharing a directory between guests. | |
| 19 | ||
| 20 | Although it is possible to use existing network file systems for some of these | |
| 21 | tasks, they require configuration steps that are hard to automate and they | |
| 22 | expose the storage network to the guest. The virtio-fs device was designed to | |
| 23 | solve these problems by providing file system access without networking. | |
| 24 | ||
| 25 | Furthermore the virtio-fs device takes advantage of the co-location of the | |
| 26 | guest and host to increase performance and provide semantics that are not | |
| 27 | possible with network file systems. | |
| 28 | ||
| 29 | Usage | |
| 30 | ===== | |
| 31 | Mount file system with tag ``myfs`` on ``/mnt``: | |
| 32 | ||
| 33 | .. code-block:: sh | |
| 34 | ||
| 35 | guest# mount -t virtiofs myfs /mnt | |
| 36 | ||
| 37 | Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU | |
| 38 | and the virtiofsd daemon. | |
| 39 | ||
| 40 | Internals | |
| 41 | ========= | |
| 42 | Since the virtio-fs device uses the FUSE protocol for file system requests, the | |
| 43 | virtiofs file system for Linux is integrated closely with the FUSE file system | |
| 44 | client. The guest acts as the FUSE client while the host acts as the FUSE | |
| 45 | server. The /dev/fuse interface between the kernel and userspace is replaced | |
| 46 | with the virtio-fs device interface. | |
| 47 | ||
| 48 | FUSE requests are placed into a virtqueue and processed by the host. The | |
| 49 | response portion of the buffer is filled in by the host and the guest handles | |
| 50 | the request completion. | |
| 51 | ||
| 52 | Mapping /dev/fuse to virtqueues requires solving differences in semantics | |
| 53 | between /dev/fuse and virtqueues. Each time the /dev/fuse device is read, the | |
| 54 | FUSE client may choose which request to transfer, making it possible to | |
| 55 | prioritize certain requests over others. Virtqueues have queue semantics and | |
| 56 | it is not possible to change the order of requests that have been enqueued. | |
| 57 | This is especially important if the virtqueue becomes full since it is then | |
| 58 | impossible to add high priority requests. In order to address this difference, | |
| 59 | the virtio-fs device uses a "hiprio" virtqueue specifically for requests that | |
| 60 | have priority over normal requests. |