1 .. SPDX-License-Identifier: GPL-2.0 2 3 .. _virtiofs_index: 4 5 =================================================== 6 virtiofs: virtio-fs host<->guest shared file system 7 =================================================== 8 9 - Copyright (C) 2019 Red Hat, Inc. 10 11 Introduction 12 ============ 13 The virtiofs file system for Linux implements a driver for the paravirtualized 14 VIRTIO "virtio-fs" device for guest<->host file system sharing. It allows a 15 guest to mount a directory that has been exported on the host. 16 17 Guests often require access to files residing on the host or remote systems. 18 Use cases include making files available to new guests during installation, 19 booting from a root file system located on the host, persistent storage for 20 stateless or ephemeral guests, and sharing a directory between guests. 21 22 Although it is possible to use existing network file systems for some of these 23 tasks, they require configuration steps that are hard to automate and they 24 expose the storage network to the guest. The virtio-fs device was designed to 25 solve these problems by providing file system access without networking. 26 27 Furthermore the virtio-fs device takes advantage of the co-location of the 28 guest and host to increase performance and provide semantics that are not 29 possible with network file systems. 30 31 Usage 32 ===== 33 Mount file system with tag ``myfs`` on ``/mnt``: 34 35 .. code-block:: sh 36 37 guest# mount -t virtiofs myfs /mnt 38 39 Please see https://virtio-fs.gitlab.io/ for details on how to configure QEMU 40 and the virtiofsd daemon. 41 42 Mount options 43 ------------- 44 45 virtiofs supports general VFS mount options, for example, remount, 46 ro, rw, context, etc. It also supports FUSE mount options. 47 48 atime behavior 49 ^^^^^^^^^^^^^^ 50 51 The atime-related mount options, for example, noatime, strictatime, 52 are ignored. The atime behavior for virtiofs is the same as the 53 underlying filesystem of the directory that has been exported 54 on the host. 55 56 Internals 57 ========= 58 Since the virtio-fs device uses the FUSE protocol for file system requests, the 59 virtiofs file system for Linux is integrated closely with the FUSE file system 60 client. The guest acts as the FUSE client while the host acts as the FUSE 61 server. The /dev/fuse interface between the kernel and userspace is replaced 62 with the virtio-fs device interface. 63 64 FUSE requests are placed into a virtqueue and processed by the host. The 65 response portion of the buffer is filled in by the host and the guest handles 66 the request completion. 67 68 Mapping /dev/fuse to virtqueues requires solving differences in semantics 69 between /dev/fuse and virtqueues. Each time the /dev/fuse device is read, the 70 FUSE client may choose which request to transfer, making it possible to 71 prioritize certain requests over others. Virtqueues have queue semantics and 72 it is not possible to change the order of requests that have been enqueued. 73 This is especially important if the virtqueue becomes full since it is then 74 impossible to add high priority requests. In order to address this difference, 75 the virtio-fs device uses a "hiprio" virtqueue specifically for requests that 76 have priority over normal requests.
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