standard ZIL: 7m40s (ZFS default)
1x SSD ZIL: 4m07s (Flash Accelerator F20)
2x SSD ZIL: 2m42s (Flash Accelerator F20)
2x SSD mirrored ZIL: 3m59s (Flash Accelerator F20)
3x SSD ZIL: 2m47s (Flash Accelerator F20)
4x SSD ZIL: 2m57s (Flash Accelerator F20)
disabled ZIL: 0m15s
(local extraction 0m0.269s)
I was not so much interested in the absolute numbers but rather in the
relative
performance differences between the standard ZIL, the SSD ZIL and the
disabled
ZIL cases.
Oh, one more comment. If you don't mirror your ZIL, and your unmirrored SSD
goes bad, you lose your whole pool. Or at least suffer data corruption.
This is not true. If ZIL device would die while pool is imported then
ZFS would start using z ZIL withing a pool and continue to operate.
On the other hand if your server would suddenly lost power and then when
you power it up later on and ZFS detects that the ZIL is broken/gone it
will require a sysadmin intervation to force the pool import and yes
possibly loose some data.
But how is it different from any other solution where your log is put on
a separate device?
Well, it is actually different. With ZFS you can still guearantee it to
be consistent on-disk while others generally can't and often you will
have to do fsck to even mount a fs in r/w...
--
Robert Milkowski
http://milek.blogspot.com
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