[pve-devel] [PATCH docs v9 3/7] add doc section for the shared filesystem virtio-fs

[Markus Frank]

Signed-off-by: Markus Frank <m.frank at proxmox.com>
---
 qm.adoc | 94 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 92 insertions(+), 2 deletions(-)

diff --git a/qm.adoc b/qm.adoc
index fa6a772..fa1de72 100644
--- a/qm.adoc
+++ b/qm.adoc
@@ -1056,6 +1056,95 @@ 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 that enables sharing a directory between host
+and guest VM. It takes advantage of the locality of virtual machines and the
+hypervisor to get a higher throughput than the 9p remote file system protocol.
+
+To use virtio-fs, the https://gitlab.com/virtio-fs/virtiofsd[virtiofsd] daemon
+needs to run in the background. In {pve}, this process starts immediately before
+the start of QEMU.
+
+Linux VMs with kernel >=5.4 support this feature 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
+^^^^^^^^^^^^^^^^^
+
+* Virtiofsd crashing means no recovery until VM is fully stopped and restarted.
+* Virtiofsd not responding may result in NFS-like hanging access in the VM.
+* Memory hotplug does not work in combination with virtio-fs (also results in
+hanging access).
+* Live migration does not work.
+* Windows cannot understand ACLs. Therefore, disable it for Windows VMs,
+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, either 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 \
+    --xattr 1 \
+    --acl 1
+----
+
+The `acl` parameter automatically implies `xattr`, that is, it makes no
+difference whether you set `xattr` to `0` if `acl` is set to `1`.
+
+Set `submounts` to `1` when multiple file systems are mounted in a shared
+directory to prevent the guest from creating duplicates because of file system
+specific inode IDs that get passed through.
+
+
+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 title "Caching Modes". 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.
+
+Additionally, it is possible to overwrite the default mapping settings for
+`xattr` and `acl` by setting them to either `1` or `0`. The `acl` parameter
+automatically implies `xattr`, that is, it makes no difference whether you set
+`xattr` to `0` if `acl` is set to `1`.
+
+----
+qm set <vmid> -virtiofs0 dirid=<dirid>,cache=always,direct-io=1
+qm set <vmid> -virtiofs1 <dirid>,cache=never,xattr=1
+qm set <vmid> -virtiofs2 <dirid>,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
 ~~~~~~~~~~~~~~~~~
@@ -1662,8 +1751,9 @@ 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
-- 
2.39.2