Message-ID: <4E255A39.4000304@2ndQuadrant.com>
Date: Tue, 19 Jul 2011 06:19:37 -0400
From: Greg Smith <greg@2ndQuadrant.com>
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To: pgsql-performance@postgresql.org
Subject: Re: BBU still needed with SSD?
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On 07/18/2011 11:56 PM, Andy wrote:
> I'm talking about after I get 2 Intel 320s, should I spend the extra 
> money on a RAID BBU? Adding RAID BBU in this case wouldn't improve 
> reliability, but does it improve performance? If so, how much 
> improvement can it bring?

It won't improve performance enough that I would bother.  The main 
benefit of adding a RAID with BBU to traditional disks is that you can 
commit much, much faster to the card RAM than the disks can spin.  You 
can go from 100 commits/second to 10,000 commits/second that way (in 
theory--actually getting >2000 at the database level is harder).

Since the Intel 320 drives can easily hit 2000 to 4000 commits/second on 
their own, using the cache that's built-in to the drive, the advantage 
of adding a RAID card on top of that is pretty minimal.  Adding a RAID 
cache will help some, because that layer will be faster than the SSD at 
absorbing writes, and putting another cache layer into a system always 
helps with improving burst performance.  But you'd probably be better 
off using the same money to add more RAM, or more/bigger SSD drives.  
The fundamental thing that RAID BBU units do--speed up commits--you will 
only see minimal benefit from with these SSDs.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD