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Hi All; We're talking with a HW / Data Center company about scaling up our DB servers... Below are some questions they asked relaed to moving to SSD's or maybe a Fusion IO drive. Anyone have any thoughts, specifically on the queue depth question? Thanks in advance... So our question I think would be:
- What queue depth do you think your postgres server
can saturate under your maximum load, when you'd want 35,000
IOPs?
Basically my concern is this: assuming a ~5ms round
trip, which probably a top end for a "good" SSD array, you
effectively get 200 IOPs per queue depth at the full 5ms
(1000ms / 5ms for 200 IOPs per QD; so 1600 IOPS for QD=16).
That means to get 35k IOPs you'd need a collective
QD=175.
An additional question would be, and Lou already
touched on this:
- Are you comfortable with being presented multiple
LUNs and striping them for your database filesystem? I'm not
100% sure about the per-LUN limits of the EMC offerings which
we've started looking into, but SolidFire tends to max out
around 17k IOPs per volume, for example; they have a stated
maximum of 15k, and when the array is under full load it's
more realistic to only expect about 8k. So to get 35k, we'd
probably want to present multiple volumes to the OS and let it
stripe them (no parity since the durability would be on the
array).
A related question would be:
- As you migrate away from RHCS, how do you feel
about utilizing local SSDs to meet your performance targets?
If your performance characteristics around your queue depth
don't fit well with an array, the lower latency to a local SSD
might make it easier to sustain 35k IOPs with local RAID SSDs.
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