Re: SSD + RAID

On Fri, Nov 13, 2009 at 9:48 AM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com> wrote:
> I think RAID6 is gonna reduce the throughput due to overhead to
> something far less than what a software RAID-10 would achieve.

I was wondering about this. I think raid 5/6 might be a better fit
for SSD than traditional drives arrays. Here's my thinking:

*) flash SSD reads are cheaper than writes. With 6 or more drives,
less total data has to be written in Raid 5 than Raid 10. The main
component of raid 5 performance penalty is that for each written
block, it has to be read first than written...incurring rotational
latency, etc. SSD does not have this problem.

*) flash is much more expensive in terms of storage/$.

*) flash (at least the intel stuff) is so fast relative to what we are
used to, that the point of using flash in raid is more for fault
tolerance than performance enhancement. I don't have data to support
this, but I suspect that even with relatively small amount of the
slower MLC drives in raid, postgres will become cpu bound for most
applications.

merlin