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List:       postgresql-general
Subject:    Re: [GENERAL] SSDs with Postgresql?
From:       Greg Smith <greg () 2ndQuadrant ! com>
Date:       2011-04-30 4:59:46
Message-ID: 4DBB9742.6070508 () 2ndQuadrant ! com
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On 04/29/2011 06:42 AM, Scott Marlowe wrote:
> I think you misunderstood.  He's not storing 480GB on the drives,
> that's how much WAL is moving across it.  It could easily be a single
> 80GB SSD drive or something like that.
>    

Right; that's why you don't necessarily get saved by the fact that 
larger databases must go onto more flash cells, too.  Sometimes, yes, 
but not always.  The WAL is really close to a worst-case for flash:  
lots of redundant information that's constantly overwritten.  It's the 
last thing you want to consider putting onto SSD.  There's a good reason 
why so many of the "enterprise" SSDs try to distinguish themselves with 
redundancy and wear leveling advancements; it's so this sort of workload 
doesn't kill them.

Combine that workload possibility with the limitations of MLC flash, and 
you can see why the lifetimes actually are a serious concern in some 
situations.  Not all of them, of course, but this is why I recommend 
things like directly measuring your WAL volume.

-- 
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


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