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Re: [linux-lvm] LVM and chain of snapshots



Dne 20.1.2016 v 14:02 Alexander E. Patrakov napsal(a):
20.01.2016 17:09, Zdenek Kabelac wrote:
Using lots of old-snaphosts is very bad plan - really time to look at
thin-provisioning....
Old-snaps are not going to scale.....

Well, I have already tried using lots of thin snapshots on my PC, in a context
different from what Mark wants. Namely, my snapshots were created by Snapper,
hourly.

Thin snapshots may indeed scale well for reads and writes, but currently they
don't scale at all for the initial activation. The problem is that "vgchange
-ay" and "vgchange -an", by default, run "thin_check", and the more snapshots
I have, the more it takes for "thin_check" to finish. With 1 snapshot, it is
quick enough so that I don't notice. But with, say, 200 snapshots (some of
which include creation of an iso image), it takes more than 1.5 minutes - more
than the initramfs (or myself) is going to wait during the boot process.

Well the execution of thin_check is 'rather' security feature.
It's been added to capture possible errors of kernel driver.
I guess now we are possibly in the age where deep checking might no longer be necessary.

So if you feel the time spend on thin_checking doesn't pay-off - you can try to add option '--skip-mappings' (see lvm.conf field global/thin_check_options)

Also  you really should not keep 200 snapshots if you don't need them.
So probably try to maintain your snapshots better (drop unneeded ones...)

And final note - I do not believe activation of multi-gigabyte old-snapshot is going to give you any better results - since reading of old metadata is actually much slower then management of thin-pool & thin_check - just try it....

It's fairly naive to expect old-snaps will do any better job when we are talking about 200 snaps.

Also there is no support for snaps of snaps of snaps with old-snapshot,
so it's not even comparable....

Regards

Zdenek



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