Provide an exmple in a place more visible than formatdomain.html. Signed-off-by: Ján Tomko <jtomko@redhat.com> Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>
4.3 KiB
Sharing files with Virtio-FS
Virtio-FS
Virtio-FS is a shared file system that lets virtual machines access a directory tree on the host. Unlike existing approaches, it is designed to offer local file system semantics and performance.
See https://virtio-fs.gitlab.io/
Host setup
Almost all virtio devices (all that use virtqueues) require access to at least certain portions of guest RAM (possibly policed by DMA). In case of virtiofsd, much like in case of other vhost-user (see https://www.qemu.org/docs/master/interop/vhost-user.html) virtio devices that are realized by an userspace process, this in practice means that QEMU needs to allocate the backing memory for all the guest RAM as shared memory. As of QEMU 4.2, it is possible to explicitly specify a memory backend when specifying the NUMA topology. This method is however only viable for machine types that do support NUMA. As of QEMU 5.0.0 and libvirt 6.9.0, it is possible to specify the memory backend without NUMA (using the so called memobject interface).
Either of the following:
Use file-backed memory
Configure the directory where the files backing the memory will be stored with the
memory_backing_dir
option in/etc/libvirt/qemu.conf
# This directory is used for memoryBacking source if configured as file. # NOTE: big files will be stored here memory_backing_dir = "/dev/shm/"
Use hugepage-backed memory
Make sure there are enough huge pages allocated for the requested guest memory. For example, for one guest with 2 GiB of RAM backed by 2 MiB hugepages:
# virsh allocpages 2M 1024
Guest setup
Specify the NUMA topology (this step is only required for the NUMA case)
in the domain XML of the guest. For the simplest one-node topology for a guest with 2GiB of RAM and 8 vCPUs:
<domain> ... <cpu ...> <numa> <cell id='0' cpus='0-7' memory='2' unit='GiB' memAccess='shared'/> </numa> </cpu> ... </domain>
Note that the CPU element might already be specified and only one is allowed.
Specify the memory backend
Either of the following:
File-backed memory
<domain> ... <memoryBacking> <access mode='shared'/> </memoryBacking> ... </domain>
This will create a file in the directory specified in
qemu.conf
Hugepage-backed memory
<domain> ... <memoryBacking> <hugepages> <page size='2' unit='M'/> </hugepages> <access mode='shared'/> </memoryBacking> ... </domain>
Add the
vhost-user-fs
QEMU device via thefilesystem
element<domain> ... <devices> ... <filesystem type='mount' accessmode='passthrough'> <driver type='virtiofs'/> <source dir='/path'/> <target dir='mount_tag'/> </filesystem> ... </devices> </domain>
Note that despite its name, the
target dir
is actually a mount tag and does not have to correspond to the desired mount point in the guest.So far,
passthrough
is the only supported access mode and it requires running thevirtiofsd
daemon as root.Boot the guest and mount the filesystem
guest# mount -t virtiofs mount_tag /mnt/mount/path
Note: this requires virtiofs support in the guest kernel (Linux v5.4 or later)
Optional parameters
More optional elements can be specified
<driver type='virtiofs' queue='1024'/>
<binary path='/usr/libexec/virtiofsd' xattr='on'>
<cache mode='always'/>
<lock posix='on' flock='on'/>
</binary>
Externally-launched virtiofsd
Libvirtd can also connect the vhost-user-fs
device to a virtiofsd
daemon launched outside of libvirtd. In that case socket permissions, the mount tag and all the virtiofsd options are out of libvirtd's control and need to be set by the application running virtiofsd.
<filesystem type='mount'/>
<driver type='virtiofs' queue='1024'/>
<source socket='/var/virtiofsd.sock'/>
</filesystem>