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
