Storage for Slicer¶

In the walkthrough, we show how to use disk images for storage.

Disk images were not the first choice for Slicer, initially, only snapshotting filesystems were supported. With a snapshot, the initial filesystem is "unpacked" once - taking anywhere between 30 and 90 seconds, then can be cloned instantly.

Disk Images - static files allocated on start-up, can take a few seconds to clone the initial disk
Snapshotting / Copy On Write (CoW) - dynamic filesystems that only store changes and allow for instant cloning

Disk images¶

Disk images are similar to loopback filesystems which you may have created in the past via fallocate and mkfs.ext4.

Pros:

Convenient, with a custom disk size
Easy to manage and migrate between systems

Cons:

No deduplication, as is possible with snapshots/Copy On Write (CoW) systems
Launch/clone time slower than CoW when launching many VMs

Resizing disk images¶

If you need more storage space in a VM using image storage mode, resize the disk after shutting down the VM:

Shutdown the VM:
```
slicer vm shutdown vm-hostname
```

Then close Slicer with Control + C, so you can boot the VM again on next start up.

Check the filesystem on the disk image:
```
sudo e2fsck -fy ./vm-hostname.img
```
Resize the filesystem to a given size i.e. 30G:
```
sudo resize2fs ./vm-hostname.img 30G
```
Start the VM again:
```
slicer up ./config.yaml
```

Note: The VM must be using image storage mode (set with storage: image in your config). This assumes you've already increased the physical disk image size beforehand.

Warning: Always run e2fsck before resizing and ensure the VM is completely shut down to avoid data corruption.

ZFS¶

ZFS is an advanced filesystem that supports CoW snapshots.

Pros:

Built-in support for snapshots and deduplication
Instant clone of base snapshot - ideal for launching many VMs
Easier to troubleshoot and understand than devmapper

Cons:

More complex to set up and manage than disk images
Requires an additional disk or partition
Additional setup required
No custom sizing - must match the base snapshot

Setup ZFS storage for Slicer

Devmapper¶

Devmapper is available for Slicer, but not set up by default and requires additional setup.

Generally, we'd recommend using ZFS instead of devmapper unless you have a specific need for it.

Pros:

Reasonably well known from the Docker space
Instant clone of base snapshot

Cons:

Requires an additional disk or partition
No custom size for VMs - the size of any VM must match the base snapshot
Difficult to debug and troubleshoot - it's easier to recreate the whole storage pool

Setup devmapper storage for Slicer