[pve-devel] [PATCH docs v13 3/12] add doc section for the shared filesystem virtio-fs

[Fiona Ebner]

Am 22.01.25 um 11:08 schrieb Markus Frank:
> Signed-off-by: Markus Frank <m.frank at proxmox.com>
> ---
>  qm.adoc | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 90 insertions(+), 2 deletions(-)
> 
> diff --git a/qm.adoc b/qm.adoc
> index 4bb8f2c..5ad79c1 100644
> --- a/qm.adoc
> +++ b/qm.adoc
> @@ -1202,6 +1202,93 @@ 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 file system 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 file system.
> +
> +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 related VM.

Nit: "related" sounds a bit vague IMHO. Maybe be explicit with something
like "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 should crash, its mount point will hang in the VM until the VM

Nit: "If Virtiofsd should crash" -> "If virtiofsd crashes"

> +is completely stopped.
> +* Virtiofsd not responding may result in NFS-like hanging access in the VM.

Nit: again, I'd not capitalize virtiofsd

"NFS-like hanging access" might not be clear to all users. Maybe
something like "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. Therefore, disable it for Windows VMs,

"ACLs" -> "ACLs in the context of virtio-fs" or "ACLs for virtio-fs mounts"

Nit: "disable it" -> "disable ACLs", never hurts to be explicit in docs

> +otherwise the virtio-fs device will not be visible within the VMs.
> +
> +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,submounts=1 \

The example still uses 'submounts' rather than 'announce-submounts'.

> +----
> +
> +Set `announce-submounts` to `1` if multiple filesystems are mounted in a shared
> +directory, to tell the guest which directories are mount points to prevent data

Nit: I'd split it in two sentences: "directory. This tells"

> +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`.
> +
> +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.
> +

I'd add a sentence describing expose-acl and expose-xattr first. And
mention again that expose-acl should not be used for Windows to make it
unlikely that user miss it.

> +The `expose-acl` parameter automatically implies `expose-xattr`, that is, it
> +makes no difference if you set `expose-xattr` to `0` if `expose-acl` is set to
> +`1`.
> +
> +----
> +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 +1972,9 @@ 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