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docs: fs: Update virtio-fs documentation

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The cloud-hypervisor supports the DAX shared region for better virtio-fs
performances, but it was not updated in the documentation. This commit
takes care of this and also update the mount command since it changed
with virtio-fs v0.3.

Fixes #433

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
This commit is contained in:
Sebastien Boeuf 2019-11-15 08:53:16 -08:00
parent 4d0872df5f
commit d9695a0fd9
1 changed files with 24 additions and 5 deletions
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29
docs/fs.md
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@ -27,9 +27,12 @@ _Run virtiofsd_
-d \
-o vhost_user_socket=/tmp/virtiofs \
-o source=/tmp/shared_dir \
-o cache=none
-o cache=always
```
The `cache=none` option here is an important one as it tells the daemon not to try any memory mapping of the files, but instead to use the _virtqueues_ to convey the files content. The support for the memory mapping of the files will be added later.
The `cache=always` option should be the default when using `virtiofsd` with the __cloud-hypervisor__ VMM. This allows the daemon to memory map the shared files, which gives better I/O performance.
The `cache=none` option is another way to run the daemon but because the _virtqueues_ are used to convey the files content in this case, the I/O performance is impacted.
### The kernel
@ -59,12 +62,28 @@ Assuming you have `clear-kvm.img` and `custom-vmlinux.bin` on your system, here
--fs tag=virtiofs,sock=/tmp/virtiofs,num_queues=1,queue_size=512
```
By default, DAX is enabled with a cache window of 8GiB. You can specify a custom size (let's say 4GiB for this example) for the cache by explicitly setting DAX and the cache size:
```bash
--fs tag=virtiofs,sock=/tmp/virtiofs,num_queues=1,queue_size=512,dax=on,cache_size=4G
```
In case you don't want to use a shared window of cache to pass the shared files content, this means you will have to explicitly disable DAX with `dax=off`. Note that in this case, the `cache_size` parameter will be ignored.
```bash
--fs tag=virtiofs,sock=/tmp/virtiofs,num_queues=1,queue_size=512,dax=off
```
### Mount the shared directory
The last step is to mount the shared directory inside the guest, using the `virtio_fs` filesystem type.
```bash
mkdir mount_dir
mount \
-t virtio_fs /dev/null mount_dir/ \
-o tag=virtiofs,rootmode=040000,user_id=0,group_id=0
-t virtio_fs virtiofs mount_dir/ \
-o rootmode=040000,user_id=0,group_id=0,dax
```
The `tag` needs to be consistent with what has been provided through the __cloud-hypervisor__ command line.
The `tag` needs to be consistent with what has been provided through the __cloud-hypervisor__ command line, which happens to be `virtiofs` in this example.
The `dax` option must be removed in case the shared cache region is not enabled from the VMM.