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Re: Testing Sandforce SSD

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Yeb Havinga <yebhavinga(at)gmail(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Testing Sandforce SSD
Date: 2010-07-24 22:01:00
Message-ID: (view raw or whole thread)
Lists: pgsql-performance
Yeb Havinga wrote:
> Writes/s start low but quickly converge to a number in the range of 
> 1200 to 1800. The writes diskchecker does are 16kB writes. Making this 
> 4kB writes does not increase writes/s. 32kB seems a little less, 64kB 
> is about two third of initial writes/s and 128kB is half.

Let's turn that into MB/s numbers:

4k * 1200 = 4.7 MB/s
8k * 1200 = 9.4 MB/s
16k * 1200 = 18.75 MB/s
64kb * 1200 * 2/3 [800] = 37.5 MB/s
128kb * 1200 / 2 [600] = 75 MB/s

For comparison sake, a 7200 RPM drive running PostgreSQL will do <120 
commits/second without a BBWC, so at an 8K block size that's <1 MB/s.  
If you put a cache in the middle, I'm used to seeing about 5000 8K 
commits/second, which is around 40 MB/s.  So this is sitting right in 
the middle of those two.  Sequential writes with a commit after each one 
like this are basically the worst case for the SSD, so if it can provide 
reasonable performance on that I'd be happy.

Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support

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