From: Greg Smith <greg@2ndQuadrant.com>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Performance advice for a new low(er)-power server
Date: Thu, 16 Jun 2011 18:12:08 -0400
Message-ID: <4DFA7FB8.80705@2ndQuadrant.com> (raw)
In-Reply-To: <BANLkTin1BZN6oAy-OJT4URJMDsMtHQ3ahA@mail.gmail.com>
References: <BANLkTinXa4ZVigTQ7Q=05saam_3GZdyx7g@mail.gmail.com>
<4DFA4EB7.1020701@2ndQuadrant.com>
<BANLkTin1BZN6oAy-OJT4URJMDsMtHQ3ahA@mail.gmail.com>
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-transa...),
> 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
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To: pgsql-performance@postgresql.org
Cc: greg@2ndQuadrant.com, mmoncure@gmail.com
Subject: Re: Performance advice for a new low(er)-power server
In-Reply-To: <4DFA7FB8.80705@2ndQuadrant.com>
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