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Re: a heavy duty operation on an "unused" table kills my server

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Andy Colson <andy(at)squeakycode(dot)net>
Cc: Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, pgsql-performance(at)postgresql(dot)org
Subject: Re: a heavy duty operation on an "unused" table kills my server
Date: 2010-01-14 18:07:44
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Andy Colson wrote:
> On 1/13/2010 11:36 PM, Craig Ringer wrote:
>> Yes. My 3ware 8500-8 on a Debian Sarge box was so awful that launching a
>> terminal would go from a 1/4 second operation to a 5 minute operation
>> under heavy write load by one writer. I landed up having to modify the
>> driver to partially mitigate the issue, but a single user on the
>> terminal server performing any sort of heavy writing would still
>> absolutely nuke performance.
> On a side note, on linux, would using the deadline scheduler resolve 
> that?

I've never seen the deadline scheduler resolve anything.  If you're out 
of I/O capacity and that's blocking other work, performance is dominated 
by the policies of the underlying controller/device caches.  Think about 
it a minute:  disks nowadays can easily have 32MB of buffer in them, 
right?  And random read/write operations are lucky to clear 2MB/s on 
cheap drivers.  So once the drive is filled with requests, you can 
easily sit there for ten seconds before the scheduler even has any input 
on resolving the situation.  That's even more true if you've got a 
larger controller cache in the mix.

Greg Smith    2ndQuadrant   Baltimore, MD
PostgreSQL Training, Services and Support

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