Re: Why is the performance of my lvmthin snapshot so poor
Zhiyong Ye <[email protected]>
| Newsgroups | gmane.linux.lvm.general |
|---|---|
| Message-ID | <[email protected]> |
在 6/14/22 10:54 PM, Gionatan Danti 写道: > 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. It seems that the consumption of COW copies when sending 4k requests is much greater than the loss from metadata. > 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. I regenerated the thin volume with the chunksize of 64K and the random write performance data tested with fio 64k requests is as follows: case iops thin lv 9381 snapshotted thin lv 8307 _______________________________________________ linux-lvm mailing list [email protected] https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/