Message-ID: <519F5909.7070000@2ndQuadrant.com>
Date: Fri, 24 May 2013 08:11:53 -0400
From: Greg Smith <greg@2ndQuadrant.com>
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Subject: Re: Reliability with RAID 10 SSD and Streaming Replication
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On 5/22/13 2:45 PM, Shaun Thomas wrote:
> That read rate and that throughput suggest 8k reads. The queue size is
> 270+, which is pretty high for a single device, even when it's an SSD.
> Some SSDs seem to break down on queue sizes over 4, and 15 sectors
> spread across a read queue of 270 is pretty hash. The drive tested here
> basically fell over on servicing a huge diverse read queue, which
> suggests a firmware issue.

That's basically it.  I don't know that I'd put the blame specifically 
onto a firmware issue without further evidence that's the case though. 
The last time I chased down a SSD performance issue like this it ended 
up being a Linux scheduler bug.  One thing I plan to do for future SSD 
tests is to try and replicate this issue better, starting by increasing 
the number of clients to at least 300.

Related:  if anyone read my "Seeking PostgreSQL" talk last year, some of 
my Intel 320 results there were understating the drive's worst-case 
performance due to a testing setup error.  I have a blog entry talking 
about what was wrong and how it slipped past me at 
http://highperfpostgres.com/2013/05/seeking-revisited-intel-320-series-and-ncq/

With that loose end sorted, I'll be kicking off a brand new round of SSD 
tests on a 24 core server here soon.  All those will appear on my blog. 
  The 320 drive is returning as the bang for buck champ, along with a DC 
S3700 and a Seagate 1TB Hybrid drive with NAND durable write cache.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com


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