[linux-lvm] Why is the performance of my lvmthin snapshot so poor

[Gionatan Danti]

Il 2022-06-13 10:49 Zhiyong Ye ha scritto:
> The performance degradation after snapshotting is expected as writing
> to a snapshotted lv involving reading the original data, writing it
> elsewhere and then writing new data into the original chunk. But the
> performance loss was so much more than I expected. Is there any way to
> improve performance after creating a snapshot? Can I ask for your
> help?

This is the key point: when first writing to a new chunk, not only it 
needs to be allocated, but old data must be copied. This r/m/w operation 
transform an async operation (write) on a sync one (read), ruining 
performance. Subsequent writes to the same chunk does have the same 
issue.

The magnitute of the slowdown seems somewhat excessive, though. When 
dealing with HDD pools, I remember a 3-5x impact on IOPs. Can you show 
the exact fio command and the parameters of your thin pool (ie: chunk 
size) and storage subsystem (HDD vs SSD, SATA vs SAS vs NVME)?

Regards.

-- 
Danti Gionatan
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