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Re: SSD + RAID

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Laszlo Nagy <gandalf(at)shopzeus(dot)com>
Cc: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: SSD + RAID
Date: 2009-11-13 17:21:28
Message-ID: 4AFD9598.2040305@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-performance
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


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  • SSD + RAID at 2009-11-13 12:46:15 from Laszlo Nagy

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