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Re: H800 + md1200 Performance problem

From: Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>
To: Tomas Vondra <tv(at)fuzzy(dot)cz>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: H800 + md1200 Performance problem
Date: 2012-04-04 16:22:58
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Lists: pgsql-performance
On Wed, Apr 4, 2012 at 7:20 AM, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:
> On 4.4.2012 15:15, Scott Marlowe wrote:
>> On Wed, Apr 4, 2012 at 3:42 AM, Cesar Martin <cmartinp(at)gmail(dot)com> wrote:
>>> I have noticed that since I changed the setting  vm.zone_reclaim_mode = 0,
>>> swap is totally full. Do you recommend me disable swap?
>> Yes
> Careful about that - it depends on how you disable it.
> Setting 'vm.swappiness = 0' is a good idea, don't remove the swap (I've
> been bitten by the vm.overcommit=2 without a swap repeatedly).

I've had far more problems with swap on and swappiness set to 0 than
with swap off.  But this has always been on large memory machines with
64 to 256G memory.  Even with fairly late model linux kernels (i.e.
10.04 LTS through 11.04) I've watched the kswapd start up swapping
hard on a machine with zero memory pressure and no need for swap.
Took about 2 weeks of hard running before kswapd decided to act

Seen it with swap on, with swappiness to 0, and overcommit to either 0
or 2 on big machines.  Once we just took the swap partitions away it
the machines ran fine.

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Subject: Re: H800 + md1200 Performance problem
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