[linux-lvm] Snapshot behavior on classic LVM vs ThinLVM

Tomas Dalebjörk tomas.dalebjork at gmail.com
Fri Apr 7 18:21:36 UTC 2017


Agent less snapshot of the vm server might be an issue with application
running in the vm guest os.
Especially as there are no VSS like features on linux.

Perhaps someone can introduce a udev listener that can be used?

Den 6 apr. 2017 16:32 skrev "Gionatan Danti" <g.danti at assyoma.it>:

> 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 at assyoma.it - info at assyoma.it
> GPG public key ID: FF5F32A8
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