On 10-12-2010 14:58 Andy wrote:
We use ZFS and use SSDs for both the log device and L2ARC. All
disks and SSDs are behind a 3ware with BBU in single disk mode.
Out of curiosity why do you put your log on SSD? Log is all
sequential IOs, an area in which SSD is not any faster than HDD. So
I'd think putting log on SSD wouldn't give you any performance
boost.
The "common knowledge" you based that comment on, may actually not be
very up-to-date anymore. Current consumer-grade SSD's can achieve up to
200MB/sec when writing sequentially and they can probably do that a lot
more consistent than a hard disk.
Have a look here: http://www.anandtech.com/show/2829/21
The sequential writes-graphs consistently put several SSD's at twice the
performance of the VelociRaptor 300GB 10k rpm disk and that's a test
from over a year old, current SSD's have increased in performance,
whereas I'm not so sure there was much improvement in platter based
disks lately?
Apart from that, I'd guess that log-devices benefit from reduced latencies.
Its actually the recommended approach from Sun to add a pair of (small
SLC-based) ssd log devices to increase performance (especially for
nfs-scenario's where a lot of synchonous writes occur) and they offer it
as an option for most of their "Unified Storage" appliances.
Best regards,
Arjen
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