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99 lines
3.0 KiB
Markdown
99 lines
3.0 KiB
Markdown
# How to use virtio-fs
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In the context of virtualization, it is always convenient to be able to share a
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directory from the host with the guest.
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__virtio-fs__, also known as __vhost-user-fs__ is a virtual device defined by
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the VIRTIO specification which allows any VMM to perform filesystem sharing.
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## Pre-requisites
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### The daemon
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This virtual device relies on the _vhost-user_ protocol, which assumes the
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backend (device emulation) is handled by a dedicated process running on the
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host. This daemon is called __virtiofsd__ and needs to be present on the host.
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_Build virtiofsd_
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```bash
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git clone https://gitlab.com/virtio-fs/virtiofsd
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pushd virtiofsd
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cargo build --release
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sudo setcap cap_sys_admin+epi target/release/virtiofsd
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```
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_Create shared directory_
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```bash
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mkdir /tmp/shared_dir
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```
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_Run virtiofsd_
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```bash
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./virtiofsd \
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--log-level debug \
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--socket-path=/tmp/virtiofs \
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--shared-dir=/tmp/shared_dir \
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--cache=never \
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--thread-pool-size=$N
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```
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The `cache=never` option is the default when using `virtiofsd` with
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Cloud Hypervisor. This prevents from using the host page cache, reducing the
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overall footprint on host memory. This increases the maximum density of virtual
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machines that can be launched on a single host.
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The `cache=always` option will allow the host page cache to be used, which can
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result in better performance for the guest's workload at the cost of increasing
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the footprint on host memory.
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The `thread-pool-size` option controls how many IO threads are spawned. For
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very fast storage like NVMe spawning enough worker threads is critical to
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getting an acceptable performance compared to native.
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### Kernel support
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Modern Linux kernels (at least v5.10) have support for virtio-fs. Use of older
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kernels, with additional patches, are not supported.
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## How to share directories with cloud-hypervisor
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### Start the VM
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Once the daemon is running, the option `--fs` from Cloud Hypervisor needs
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to be used.
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Both direct kernel boot and EFI firmware can be used to boot a VM with
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virtio-fs, given that the cloud image contains a recent enough kernel.
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Correct functioning of `--fs` requires `--memory shared=on` to facilitate
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interprocess memory sharing.
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Assuming you have `focal-server-cloudimg-amd64.raw` and `vmlinux` on your
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system, here is the Cloud Hypervisor command you need to run:
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```bash
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./cloud-hypervisor \
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--cpus boot=1 \
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--memory size=1G,shared=on \
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--disk path=focal-server-cloudimg-amd64.raw \
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--kernel vmlinux \
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--cmdline "console=hvc0 root=/dev/vda1 rw" \
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--fs tag=myfs,socket=/tmp/virtiofs,num_queues=1,queue_size=512
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```
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### Mount the shared directory
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The last step is to mount the shared directory inside the guest, using the
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`virtiofs` filesystem type.
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```bash
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mkdir mount_dir
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mount -t virtiofs myfs mount_dir/
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```
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The `tag` needs to be consistent with what has been provided through the
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Cloud Hypervisor command line, which happens to be `myfs` in this example.
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## DAX feature
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Given the DAX feature is not stable yet from a daemon standpoint, it is not
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available in Cloud Hypervisor.
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