Re: SSD + RAID

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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@xxxxxxxxxxxxxxx  www.2ndQuadrant.com


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