ZFS Storage Pools for Slicer¶

ZFS is an advanced filesystem that was originally developed at Sun Microsystems. The modern equivalent for Linux was ported as the OpenZFS project and has a license that is incompatible with the Linux Kernel, for that reason, it is kept out of tree, and must be installed separately.

The containerd project already has a ZFS snapshotter, however it is unsuitable for use for VMs, therefore we implemented our own snapshotter plugin which can present ZFS volumes as block devices to a microVM.

Installation¶

To setup ZFS you can run the installation script again, but this time, with additional flags.

Install a drive, or make a partition available for zfs to use. The installation script will automatically creatre a loopback device if no device is provided.

While using a loopback file is supported, it is absolutely not recommended to use a loopback file for ZFS with Slicer.

Run the installation script again and set the --zfs flag:

curl -sLS https://get.slicervm.com | sudo bash -s -- \
  --zfs /dev/nvme0n1 \
  --overwrite # Destroy any existing content on the disk

Be very careful that you specify the correct drive or partition. This operation cannot be reversed and will destroy any existing contents.

The default size for any unpacked VM disk is 30GB. See adjust the base snapshot size to change this.

Use ZFS for VM storage¶

Let's customise the walkthrough example for ZFS.

1) Change the storage type from image to zfs:

config:
  host_groups:
  - name: vm
-   storage: image
+   storage: zfs
-   storage_size: 20G

See the note below on storage_size.

2) Customise the storage_size

The storage_size field is optional for ZFS.

If not specified, the default size of the base snapshot will be used. A custom size can be given, so long as it is equal to or larger than the base snapshot size.

config:
  host_groups:
  - name: vm
    storage: zfs
+   storage_size: 40G

If the base snapshot size is large for any existing VMs, then you can find its lease and remove it before having it re-created with the new settings for the vzol-snapshotter.

$ sudo ctr -n slicer leases ls
ID            CREATED AT           LABELS 
slicer/k3s-1  2025-09-04T13:53:07Z -

Then, find the lease ID for the VM in question, and delete it. The lease ID is the hostname of the VM i.e.k3s-1.

sudo ctr -n slicer leases rm slicer/k3s-1

To delete all leases:

sudo ctr -n slicer leases ls -q | xargs -n1 sudo ctr -n slicer leases rm

Adjust the base snapshot size¶

The base snapshot size can be changed by updating the snapshooter configuration file and restarting the zvol snapshotter service.

Edit /etc/containerd-zvol-grpc/config.toml and replace the volume size with the desired value, e.g 40G:

root_path="/var/lib/containerd-zvol-grpc"
dataset="your-zpool/snapshots"
-volume_size="30G"
+volume_size="40G"
fs_type="ext4"

Finally restart the zvol-snapshotter service:

sudo systemctl restart zvol-snapshotter