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Re: SSD + RAID

From: Dan Langille <dan(at)langille(dot)org>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Matthew Wakeling <matthew(at)flymine(dot)org>, Greg Smith <greg(at)2ndquadrant(dot)com>, Laszlo Nagy <gandalf(at)shopzeus(dot)com>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: SSD + RAID
Date: 2010-02-20 23:07:16
Message-ID: 4B806B24.40503@langille.org (view raw or flat)
Thread:
Lists: pgsql-performance
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Bruce Momjian wrote:
> Matthew Wakeling wrote:
>> On Fri, 13 Nov 2009, Greg Smith wrote:
>>> 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.
>> Seagate are claiming to be on the ball with this one.
>>
>> http://www.theregister.co.uk/2009/12/08/seagate_pulsar_ssd/
> 
> I have updated our documentation to mention that even SSD drives often
> have volatile write-back caches.  Patch attached and applied.

Hmmm.  That got me thinking: consider ZFS and HDD with volatile cache.
Do the characteristics of ZFS avoid this issue entirely?

- --
Dan Langille

BSDCan - The Technical BSD Conference : http://www.bsdcan.org/
PGCon  - The PostgreSQL Conference:     http://www.pgcon.org/
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