[linux-lvm] Why is the performance of my lvmthin snapshot so poor
g.danti at assyoma.it
Tue Jun 14 14:54:28 UTC 2022
Il 2022-06-14 15:29 Zhiyong Ye ha scritto:
> The reason for this may be that when the volume creates a snapshot,
> each write to an existing block will cause a COW (Copy-on-write), and
> the COW is a copy of the entire data block in chunksize, for example,
> when the chunksize is 64k, even if only 4k of data is written, the
> entire 64k data block will be copied. I'm not sure if I understand
> this correctly.
Yes, in your case, the added copies are lowering total available IOPs.
But note how the decrease is sub-linear (from 64K to 1M you have a 16x
increase in chunk size but "only" a 10x hit in IOPs): this is due to the
lowered metadata overhead.
A last try: if you can, please regenerate your thin volume with 64K
chunks and set fio to execute 64K requests. Lets see if LVM is at least
smart enough to avoid coping a to-be-completely-overwritten chunks.
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