On Jan 9, 2010, at 1:32 AM, Lutz Schumann wrote:
Depends.
a) Pool design
5 x SSD as raidZ = 4 SSD space - read I/O performance of one drive
Adding 5 cheap 40 GB L2ARC device (which are pooled) increases the
read performance for your working window of 200 GB.
An interesting thing happens when an app suddenly has 50-100x more IOPS.
The bottleneck tends to move back to the CPU. This is a good thing,
because
the application running on a CPU is where the most value is gained. Be
aware
of this, because it is not uncommon for people to upgrade the storage
and not
see significant improvement when the application becomes CPU-bound.
If you have a pool of mirrors - adding L2ARC does not make sence.
I think this is a good rule of thumb.
-- richard
b) SSD type
Is your devices are MLC adding a ZIL makes sence. Watch out for
drive qualification ! (honor the cache flush command).
Robert
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