[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM
g.danti at assyoma.it
Thu Apr 6 14:31:31 UTC 2017
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
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
Thank you all.
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
GPG public key ID: FF5F32A8
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