On 23/10/19 14:59, Zdenek Kabelac wrote:
Dne 23. 10. 19 v 13:08 Gionatan Danti napsal(a):
Talking about thin snapshot, an obvious performance optimization which
seems to not be implemented is to skip reading source data when
overwriting in larger-than-chunksize blocks.
Hi
There is no such optimization possible for old snapshots.
You would need to write ONLY to snapshots.
As soon as you start to write to origin - you have to 'read' original
data from origin, copy them to COW storage, once this is finished, you can
overwrite origin data area with your writing I/O.
This is simply never going to work fast ;) - the fast way is thin-pool...
Old snapshots were designed for 'short' lived snapshots (so you can take
a backup of volume which is not being modified underneath).
Any idea of improving this old snapshots target are sooner or later
going to end-up with thin-pool anyway :) (we've been in this river many
many years back in time...)
For example, consider a completely filled 64k chunk thin volume (with
thinpool having ample free space). Snapshotting it and writing a 4k
block on origin
There is no support of snapshot of snapshot with old snaps...
It would be extremely slow to use...
However, my testing shows that source chunks are always read, even
when completely overwritten.
Am I missing something?
Yep - you would need to always jump to your 'snapshot' - so instead of
keeping 'origin' on major:minor - it would need to become a 'snapshot'...
Seriously complex concept to work with - especially when there is
thin-pool...
Hi, I was speaking about *thin* snapshots here. Rewriting the example
given above (for clarity):
"For example, consider a completely filled 64k chunk thin volume (with
thinpool having ample free space). Snapshotting it and writing a 4k
block on origin will obviously cause a read of the original 64k chunk,
an in-memory change of the 4k block and a write of the entire modified
64k block to a new location. But writing, say, a 1 MB block should *not*
cause the same read on source: after all, the read data will be
immediately discarded, overwritten by the changed 1 MB block."
I would expect that such large-block *thin* snapshot rewrite behavior
would not cause a read/modify/write, but it really does.
Is this a low-hanging fruit or there are more fundamental problem
avoiding read/modify/write in this case?
Thanks.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
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