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shares.md

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Shares

Persistent file-systems are provided by both volumes and shares. Volumes are block devices inside the virtual machine, yielding fast performance but mounted file-systems require exclusive access. Shares allow mounting an arbitrary directory tree from the host.

In microvm.shares elements the proto field allows either of two values:

  • 9p (default) is built into many hypervisors, allowing you to quickly share a directory tree

  • virtiofs requires a separate virtiofsd service which is started as a prerequisite when you start MicroVMs through a systemd service that comes with the microvm.nixosModules.host module.

    If you want to run from the command-line, start bin/virtiofsd-run separately.

    Expect virtiofs to yield better performance over 9p.

microvm.shares = [ {
  proto = "virtiofs";
  tag = "home";
  # Source path can be absolute or relative
  # to /var/lib/microvms/$hostName
  source = "home";
  mountPoint = "/home";
} ];
When sharing a path that is on ZFS with virtiofs, the dataset must have options -o xattr=sa -o acltype=posixacl

Sharing a host's /nix/store

If a share with source = "/nix/store" is defined, size and build time of the stage1 squashfs for /dev/vda will be reduced drastically.

microvm.shares = [ {
  tag = "ro-store";
  source = "/nix/store";
  mountPoint = "/nix/.ro-store";
} ];

Writable /nix/store overlay

An optional writable layer will be mounted if the path microvm.writableStoreOverlay is set. Make sure that the path is located on a writable filesystem.

Caveat: The Linux overlay filesystem is very picky about the filesystems that can be the upper (writable) layer. 9p/virtiofs shares don't work currently, so resort to using a volume for that:

{ config, ... }:
{
  microvm.writableStoreOverlay = "/nix/.rw-store";

  microvm.volumes = [ {
    image = "nix-store-overlay.img";
    mountPoint = config.microvm.writableStoreOverlay;
    size = 2048;
  } ];
}
The Nix database will forget all built packages after a reboot, containing only what is needed for the VM's NixOS system. Until this has been solved, it is recommended to just delete and recreate the overlay after MicroVM shutdown or before startup.