[OpenIndiana-discuss] ZIL write cache performance

[Matt Connolly]

Hi, I've installed an SSD drive in my OI machine and have it partitioned (sliced) with a main slice to boot from and a smaller slice to use as a write cache (ZIL) for our data pool.

I've noticed that for many tasks, using the ZIL actually slows many tasks at hand (operation within a qemu-kvm virtual machine, mysql loading importing a dump file, etc). I know I bought a cheap SSD to play with so I wasn't expected the best performance, but I would have expected some improvement, not a slow down.

In one particular test, I have mysql running in a zone and loading a test data set takes about 40 seconds without the ZIL and about 60 seconds with ZIL. I certainly wasn't expecting a 50% slow down.

Is this to be expected?

Are there any best practices for testing an SSD to see if it will actually improve performance of a zfs pool?


Thanks,
Matt