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Re: SSDs with Postgresql?

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: "Henry C(dot)" <henka(at)cityweb(dot)co(dot)za>
Cc: Leonardo Francalanci <m_lists(at)yahoo(dot)it>, pgsql-general(at)postgresql(dot)org
Subject: Re: SSDs with Postgresql?
Date: 2011-04-14 19:14:11
Message-ID: 4DA74783.4010804@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-general
Henry C. wrote:
> I believe this perception that SSDs are less "safe" than failure-prone
> mechanical hard drives will eventually change.
>   

Only because the manufacturers are starting to care about write 
durability enough to include the right hardware for it.  Many of them 
are less safe right now on some common database tasks.  Intel's gen 1 
and gen 2 drives are garbage for database use.  I've had customers lose 
terabytes of data due to them.  Yes, every system can fail, but these 
*will* fail and corrupt your database the first time there's a serious 
power problem of some sort.  And the idea that a UPS is sufficient to 
protect against that even happening in wildly optimistic.

See http://wiki.postgresql.org/wiki/Reliable_Writes for more background 
here, and links to reading on the older Intel drives.  I summarized the 
situation with their newer 320 series drives at 
http://blog.2ndquadrant.com/en/2011/04/intel-ssd-now-off-the-sherr-sh.html  
Those finally get the write flushing right.  But the random seeks IOPS 
is wildly lower than you might expect on read/write workloads.  My own 
tests and other sources have all come up with around 3500 IOPS as being 
a real-world expectation for the larger sizes of these drives.  Also, it 
is cheap flash, so durability in a server environment won't be great.  
Don't put your WAL on them if you have a high transaction rate.  Put 
some indexes there instead.

-- 
Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)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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