Message-ID: <4E003B44.9090005@2ndQuadrant.com>
Date: Tue, 21 Jun 2011 02:33:40 -0400
From: Greg Smith <greg@2ndQuadrant.com>
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To: pgsql-performance@postgresql.org
Subject: Re: Contemplating SSD Hardware RAID
References: <4E0015F2.9060504@drivefaster.net>
In-Reply-To: <4E0015F2.9060504@drivefaster.net>
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On 06/20/2011 11:54 PM, Dan Harris wrote:
> I understand that the majority of consumer grade SSD drives lack the 
> required capacitor to complete a write on a sudden power loss.  But, 
> what about pairing up with a hardware controller with BBU write 
> cache?  Can the write cache be disabled at the drive and result in a 
> safe setup?

Sometimes, but not always, and you'll be playing a risky and 
unpredictable game to try it.  See 
http://wiki.postgresql.org/wiki/Reliable_Writes for some anecdotes.  And 
even if the reliability works out, you'll kill the expected longevity 
and performance of the drive.

> I'm exploring the combination of an Areca 1880ix-12 controller with 6x 
> OCZ Vertex 3 V3LT-25SAT3 2.5" 240GB SATA III drives in RAID-10.  Has 
> anyone tried this combination?  What nasty surprise am I overlooking 
> here?

You can expect database corruption the first time something unexpected 
interrupts the power to the server.  That's nasty, but it's not 
surprising--that's well documented as what happens when you run 
PostreSQL on hardware with this feature set.  You have to get a Vertex 3 
Pro to get one of the reliable 3rd gen designs from them with a 
supercap.  (I don't think those are even out yet though)  We've had 
reports here of the earlier Vertex 2 Pro being fully stress tested and 
working out well.  I wouldn't even bother with a regular Vertex 3, 
because I don't see any reason to believe it could be reliable for 
database use, just like the Vertex 2 failed to work in that role.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us
"PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books