Dne 23. 10. 19 v 13:08 Gionatan Danti napsal(a):
On 23/10/19 12:46, Zdenek Kabelac wrote:
Just few 'comments' - it's not really comparable - the efficiency of
thin-pool metadata outperforms old snapshot in BIG way (there is no point to
talk about snapshots that takes just couple of MiB)
Yes, this matches my experience.
There is also BIG difference about the usage of old snapshot origin and
snapshot.
COW of old snapshot effectively cuts performance 1/2 if you write to origin.
If used without non-volatile RAID controller, 1/2 is generous - I measured
performance as low as 1/5 (with fat snapshot).
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...
Regards
Zdenek
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