[pve-devel] [PATCH docs v14 3/12] add doc section for the shared filesystem virtio-fs

[Laurențiu Leahu-Vlăducu]

I think the docs should also - at least shortly - mention that:

1. The directory is only mounted temporarily and does not persist after 
a restart.
2. The reasoning behind this decision.
3. How to mount permanently, if still desired (despite the reasoning 
from point 2).


Other popular virtualization software also allow automatically mounting 
such things, so I think it makes sense to at least explain how to do it, 
even if we don't do it by default.


Some further comments inline


Otherwise, please consider:

Reviewed-by: Laurențiu Leahu-Vlăducu <l.leahu-vladucu at proxmox.com>


On 04.03.25 12:57, Markus Frank wrote:
> Signed-off-by: Markus Frank <m.frank at proxmox.com>
> ---
> v14:
> * addressed formulation nits
> * added paragraph about expose-acl & expose-xattr
> 
>   qm.adoc | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 97 insertions(+), 2 deletions(-)
> 
> diff --git a/qm.adoc b/qm.adoc
> index 4bb8f2c..86b3877 100644
> --- a/qm.adoc
> +++ b/qm.adoc
> @@ -1202,6 +1202,100 @@ recommended to always use a limiter to avoid guests using too many host
>   resources. If desired, a value of '0' for `max_bytes` can be used to disable
>   all limits.
>   
> +[[qm_virtiofs]]
> +Virtio-fs
> +~~~~~~~~~
> +
> +Virtio-fs is a shared filesystem designed for virtual environments. It allows to
> +share a directory tree available on the host by mounting it within VMs. It does
> +not use the network stack and aims to offer similar performance and semantics as
> +the source filesystem.
> +
> +To use virtio-fs, the https://gitlab.com/virtio-fs/virtiofsd[virtiofsd] daemon
> +needs to run in the background. This happens automatically in {pve} when
> +starting a VM using a virtio-fs mount.
> +
> +Linux VMs with kernel >=5.4 support virtio-fs by default.
> +
> +There is a guide available on how to utilize virtio-fs in Windows VMs.
> +https://github.com/virtio-win/kvm-guest-drivers-windows/wiki/Virtiofs:-Shared-file-system
> +
> +Known Limitations
> +^^^^^^^^^^^^^^^^^
> +
> +* If virtiofsd crashes, its mount point will hang in the VM until the VM
> +is completely stopped.
> +* virtiofsd not responding may result in a hanging mount in the VM, similar to
> +an unreachable NFS.
> +* Memory hotplug does not work in combination with virtio-fs (also results in
> +hanging access).
> +* Memory related features such as live migration, snapshots, and hibernate are
> +not available with virtio-fs devices.
> +* Windows cannot understand ACLs in the context of virtio-fs. Therefore, do not
> +expose ACLs for Windows VMs, otherwise the virtio-fs device will not be
> +visible within the VM.
> +
> +Add Mapping for Shared Directories
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +To add a mapping for a shared directory, you can use the API directly with
> +`pvesh` as described in the xref:resource_mapping[Resource Mapping] section:
> +
> +----
> +pvesh create /cluster/mapping/dir --id dir1 \
> +    --map node=node1,path=/path/to/share1 \
> +    --map node=node2,path=/path/to/share2,announce-submounts=1 \
> +----
> +
> +Set `announce-submounts` to `1` if multiple filesystems are mounted in a shared
> +directory. This tells the guest which directories are mount points to prevent
> +data loss/corruption. With `announce-submounts`, virtiofsd reports a different
> +device number for each submount it encounters. Without it, duplicates may be
> +created because inode IDs are only unique on a single filesystem.
> +
> +Add virtio-fs to a VM
> +^^^^^^^^^^^^^^^^^^^^^
> +
> +To share a directory using virtio-fs, add the parameter `virtiofs<N>` (N can be
> +anything between 0 and 9) to the VM config and use a directory ID (dirid) that
> +has been configured in the resource mapping. Additionally, you can set the
> +`cache` option to either `always`, `never`, or `auto` (default: `auto`),
> +depending on your requirements. How the different caching modes behave can be
> +read at https://lwn.net/Articles/774495/ under the "Caching Modes" section. To
> +enable writeback cache set `writeback` to `1`.

While I know that the linked article/patch contains a lot of 
information, unless we have a very good reason, I think we should either 
summarize that information in our own documentation, or we should create 
a separate section in our documentation that explains the information 
available there. The reason of that is that our documentation might be 
accessed from offline (air-gapped) machines that won't be able to access 
the internet.

> +
> +Virtiofsd supports ACL and xattr passthrough (can be enabled with the
> +`expose-acl` and `expose-xattr` options), allowing the guest to access ACLs and
> +xattrs if the underlying host filesystem supports them, but they must also be
> +compatible with the guest filesystem (for example most Linux filesystems support
> +ACLs, while Windows filesystems do not).
> +
> +The `expose-acl` option automatically implies `expose-xattr`, that is, it makes
> +no difference if you set `expose-xattr` to `0` if `expose-acl` is set to `1`.
> +
> +If you want virtio-fs to honor the `O_DIRECT` flag, you can set the `direct-io`
> +parameter to `1` (default: `0`). This will degrade performance, but is useful if
> +applications do their own caching.
> +
> +----
> +qm set <vmid> -virtiofs0 dirid=<dirid>,cache=always,direct-io=1
> +qm set <vmid> -virtiofs1 <dirid>,cache=never,expose-xattr=1
> +qm set <vmid> -virtiofs2 <dirid>,expose-acl=1,writeback=1
> +----
> +
> +To mount virtio-fs in a guest VM with the Linux kernel virtio-fs driver, run the
> +following command inside the guest:
> +
> +----
> +mount -t virtiofs <mount tag> <mount point>
> +----
> +
> +The dirid associated with the path on the current node is also used as the mount
> +tag (name used to mount the device on the guest).
> +
> +For more information on available virtiofsd parameters, see the
> +https://gitlab.com/virtio-fs/virtiofsd[GitLab virtiofsd project page].
> +
>   [[qm_bootorder]]
>   Device Boot Order
>   ~~~~~~~~~~~~~~~~~
> @@ -1885,8 +1979,9 @@ in the relevant tab in the `Resource Mappings` category, or on the cli with
>   
>   [thumbnail="screenshot/gui-datacenter-mapping-pci-edit.png"]
>   
> -Where `<type>` is the hardware type (currently either `pci` or `usb`) and
> -`<options>` are the device mappings and other configuration parameters.
> +Where `<type>` is the hardware type (currently either `pci`, `usb` or
> +xref:qm_virtiofs[dir]) and `<options>` are the device mappings and other
> +configuration parameters.
>   
>   Note that the options must include a map property with all identifying
>   properties of that hardware, so that it's possible to verify the hardware did