From: Dan Langille <dan@langille.org>
To: Bruce Momjian <bruce@momjian.us>
Cc: Matthew Wakeling <matthew@flymine.org>
Cc: Greg Smith <greg@2ndquadrant.com>
Cc: Laszlo Nagy <gandalf@shopzeus.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
Date: Sat, 20 Feb 2010 18:07:16 -0500
Message-ID: <4B806B24.40503@langille.org> (raw)
In-Reply-To: <201002201828.o1KIStQ12830@momjian.us>
References: <201002201828.o1KIStQ12830@momjian.us>
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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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To: pgsql-performance@postgresql.org
Cc: dan@langille.org, bruce@momjian.us, matthew@flymine.org, greg@2ndquadrant.com, gandalf@shopzeus.com
Subject: Re: SSD + RAID
In-Reply-To: <4B806B24.40503@langille.org>
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