Re: SSD + RAID

In order for a drive to work reliably for database use such as for
PostgreSQL, it cannot have a volatile write cache. You either need a
write cache with a battery backup (and a UPS doesn't count), or to turn
the cache off. The SSD performance figures you've been looking at are
with the drive's write cache turned on, which means they're completely
fictitious and exaggerated upwards for your purposes. In the real
world, that will result in database corruption after a crash one day.
No one on the drive benchmarking side of the industry seems to have
picked up on this, so you can't use any of those figures. I'm not even
sure right now whether drives like Intel's will even meet their lifetime
expectations if they aren't allowed to use their internal volatile write
cache.

Here's two links you should read and then reconsider your whole design:

http://www.mysqlperformanceblog.com/2009/03/02/ssd-xfs-lvm-fsync-write-cache-barrier-and-lost-transactions/
http://petereisentraut.blogspot.com/2009/07/solid-state-drive-benchmarks-and-write.html

I can't even imagine how bad the situation would be if you decide to
wander down the "use a bunch of really cheap SSD drives" path; these
things are barely usable for databases with Intel's hardware. The needs
of people who want to throw SSD in a laptop and those of the enterprise
database market are really different, and if you believe doom
forecasting like the comments at
http://blogs.sun.com/BestPerf/entry/oracle_peoplesoft_payroll_sun_sparc
that gap is widening, not shrinking.

--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com