Message-ID: <4B4F5F45.105@squeakycode.net>
Date: Thu, 14 Jan 2010 12:15:33 -0600
From: Andy Colson <andy@squeakycode.net>
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To: Greg Smith <greg@2ndquadrant.com>
CC: Craig Ringer <craig@postnewspapers.com.au>,
	pgsql-performance@postgresql.org
Subject: Re: a heavy duty operation on an "unused" table kills my
  server
References: <e24c1d9d1001122059ic5d4721o98ccf85033b1fead@mail.gmail.com>	
	<4B4D5698.8040808@emolecules.com>	
	<e24c1d9d1001122147l1c0e2111kce25dc2c7f393087@mail.gmail.com>	
	<4B4D61DA.7080008@postnewspapers.com.au>	
	<e24c1d9d1001122303g1d4eea48h18bd73467d1fa8ec@mail.gmail.com>
	<603c8f071001130823l69804fe3m5c6fbb081c55c265@mail.gmail.com>
	<4B4EAD76.9030305@postnewspapers.com.au>
	<4B4F2F00.7070401@squeakycode.net>
	<4B4F5D70.4020103@2ndquadrant.com>
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On 1/14/2010 12:07 PM, Greg Smith wrote:
> 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.
>

That makes sense.  So if there is very little io, or if there is way way 
too much, then the scheduler really doesn't matter.  So there is a slim 
middle ground where the io is within a small percent of the HD capacity 
where the scheduler might make a difference?

-Andy