Snapshot as volume chain for file level storage, use cases?

[VictorSTS]

Hello,

PVE 9 added "Support for snapshots as volume chains on Directory/NFS/CIFS storages (technology preview)". On any file level storage, like a directory or an NFS, you can use QCOW2 format for the disk(s) of your VMs, which already provides snapshot functionality and in some aspects in a better way than volume-chain. Also, QCOW2 disks can only be used in VMs: LXC still can't get snapshots on file level storages even with the volume-chain feature.

Which would be the use case for using snapshot as volume chain on file level access storage?
What am I missing here?

Thanks!

 

[bkry]

Hello,
This is a bit old thread, but here I am sharing what I know about this topic:

On any file level storage, like a directory or an NFS, you can use QCOW2 format for the disk(s) of your VMs, which already provides snapshot functionality and in some aspects in a better way than volume-chain.

You are correct, file level storage you wouldn't need to enable this feature [1].
However for block storage, more precisely LVM on top of an iSCSI or FC-based storage this feature is needed.
By default, LVM does not support VM snapshots [1]. That said, as you noted, starting with Proxmox VE 9, the snapshot-as-volume-chain feature has been introduced for virtual machines [2]. However, there are some caveats to consider. For instance, All disk images must be snapshot-able. For more details, see the description of snapshot-as-volume-chain in the LVM configuration section [3].

Hope this helps!

[1] https://pve.proxmox.com/pve-docs/chapter-pvesm.html#_storage_types
[2] https://pve.proxmox.com/wiki/Migrate_to_Proxmox_VE#Storage_boxes_(SAN/NAS)
[3] https://pve.proxmox.com/pve-docs/pve-admin-guide.html#pvesm_lvm_config

 

[VictorSTS]

I'm fully aware about the usefulness of snapshots as volume chains for LVM and I am about not needing it on any file based storage. That's not what I'm asking. My question is what is the use case and motivation to use snapshots as volume chains on file based storage when there are proven technologies to do so (qcow2 disks).

 

[aaron]

AFAIK, snapshots as volume chains can be useful on the file system level because reverting to/deleting a snapshot in a QCOW2 image needs to halt the IO to the image at some point for more or less time. That is avoided if you use snapshot volume chains.

I haven't touched that part in detail, so others will be able to offer more detailed insights.