[OmniOS-discuss] slog limits write speed more than it should

[Rune Tipsmark]

Hi all,

Got a problem... with my pool using sync=always I see a max write speed of about 6000 IOPS (64KB block size) during storage vMotion.
I doesn't matter if I have one, two or three SLOGs, if  I use one it will just do ~6000 w/s, 0% busy (SLC IO Drive), if I use two of these each will do ~3000 w/s or so... if I use 3  each will  do ~2000 w/s or so... the underlying disks in the pool are maybe 10-15% busy tops...
If I disable sync I see a tasks such as vMotion for a 40GB vm drop from 3-4 minutes to less than 1 minute.

I can't see  where the problem is, each SLOG device can easily handle much more than 6000 IOPS and vSphere can easily get vMotion done faster when sync is disabled.... So essentially SLOGs aren't busy, disks aren't busy... sync just kills the speed... what parameters can I  tweak to push this performance up significantly? I tried 512B, 4KB, 64KB and 128KB block sizes, 64KB (default) seems to have the best performance.

Any ideas welcome.
Br,
Rune



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