Message-ID: <4DFA7FB8.80705@2ndQuadrant.com>
Date: Thu, 16 Jun 2011 18:12:08 -0400
From: Greg Smith <greg@2ndQuadrant.com>
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To: Merlin Moncure <mmoncure@gmail.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Performance advice for a new low(er)-power server
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On 06/16/2011 03:04 PM, Merlin Moncure wrote:
> I don't necessarily agree. the drives are SLC and have the potential
> to have a much longer lifespan than any MLC drive, although this is
> going to depend a lot on the raid controller if write caching is
> disabled.  Also, reading the post that got all this started
> (http://www.mysqlperformanceblog.com/2009/03/02/ssd-xfs-lvm-fsync-write-cache-barrier-and-lost-transactions/),
> the OP was able to configure them to run durably with 1200 write iops.
>    

We've also seen 
http://petereisentraut.blogspot.com/2009/07/solid-state-drive-benchmarks-and-write.html 
where Peter was only able to get 441 seeks/second on the bonnie++ mixed 
read/write test that way.  And no one has measured the longevity of the 
drive when it's running in this mode.  A large portion of the lifespan 
advantage MLC would normally have over SLC goes away if it can't cache 
writes anymore.  Worst-case, if the drive is always hit with 8K writes 
and the erase size is 128KB, you might get only 1/16 of the specified 
lifetime running it cacheless.

I just can't recommend that people consider running one of these in a 
mode it was never intended to.  The fact that the consumer drives from 
this generation still lose data even with the write cache turned off 
should make you ask what other, more difficult to trigger failure modes 
are still hanging around the enterprise drives, too.  Everyone I've seen 
suffer through problems with these gave up on them before even trying 
really in-depth reliability tests, so I don't consider that territory 
even very well explored.

-- 
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