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From: Tomas Vondra <tv@fuzzy.cz>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: H800 + md1200 Performance problem
Date: Thu, 05 Apr 2012 22:06:49 +0200
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On 5.4.2012 20:43, Merlin Moncure wrote:
> The original problem is read based performance issue though and this
> will not have any affect on that whatsoever (although it's still
> excellent advice).  Also dd should bypass the o/s buffer cache.  I
> still pretty much convinced that there is a fundamental performance
> issue with the raid card dell needs to explain.

Well, there are two issues IMHO.

1) Read performance that's not exactly as good as one'd expect from a
   12 x 15k SAS RAID10 array. Given that the 15k Cheetah drives usually
   give like 170 MB/s for sequential reads/writes. I'd definitely
   expect more than 533 MB/s when reading the data. At least something
   near 1GB/s (equal to 6 drives).

   Hmm, the dd read performance seems to grow over time - I wonder if
   this is the issue with adaptive read policy, as mentioned in the
   xbitlabs report.

   Cesar, can you set the read policy to a 'read ahead'

     megacli -LDSetProp RA -LALL -aALL

   or maybe 'no read-ahead'

     megacli -LDSetProp NORA -LALL -aALL

   It's worth a try, maybe it somehow conflicts with the way kernel
   handles read-ahead or something. I find these adaptive heuristics
   a bit unpredictable ...

   Another thing - I see the patrol reads are enabled. Can you disable
   that and try how that affects the performance?

2) Write performance behaviour, that's much more suspicious ...

   Not sure if it's related to the read performance issues.

Tomas



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