[linux-lvm] LVM and chain of snapshots

[Zdenek Kabelac]

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