Hi all,
I'm seeking some advice for a new virtualization system (KVM) on top of LVM. The goal is to take agentless backups via LVM snapshots.

In short: what you suggest to snapshot a quite big (8+ TB) volume? Classic LVM (with old snapshot behavior) or thinlvm (and its new snapshot method)?

Long story:
In the past, I used classical, preallocated logical volumes directly exported as virtual disks. In this case, I snapshot the single LV I want to backup and, using dd/ddrescue, I copy it.

Problem is this solution prevents any use of thin allocation or sparse files, so I tried to replace it with something filesystem-based. Lately I used another approach, configuring a single thinly provisioned LV (with no zeroing) + XFS + raw or qcow2 virtual machine images. To make backups, I snapshotted the entire thin LV and, after mounting it, I copied the required files.

So far this second solution worked quite well. However, before using it in more and more installations, I wonder if it is the correct approach or if something better, especially from a stability standpoint, is possible.

Gived that I would like to use XFS, and that I need snapshots at the block level, two possibilities came to mind:

1) continue to use thinlvm + thin snapshots + XFS. What do you think about a 8+ TB thin pool/volume with relatively small (64/128KB) chunks? Would you be comfortable using it in production workloads? What about powerloss protection? From my understanding, thinlvm passes flushes anytime the higher layers issue them and so should be reasonable safe against unexpected powerloss. Is this view right?

2) use a classic (non-thin) LVM + normal snapshot + XFS. I know for sure that LV size is not an issue here, however big snapshot size used to be problematic: the CoW table had to be read completely before the snapshot can be activated. Is this problem a solved one? Or big snapshot can be problematic?

Thank you all.

--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

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