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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