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.
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.
However, my testing shows that source chunks are always read, even when
completely overwritten.
Am I missing something?
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
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