Re: postgresql.conf recommendations

I've benchmarked shared_buffers with high and low settings, in a server dedicated to postgres with 48GB my settings are:
shared_buffers = 37GB
effective_cache_size = 38GB

Having a small number and depending on OS caching is unpredictable, if the server is dedicated to postgres you want make sure postgres has the memory. A random unrelated process doing a cat /dev/sda1 should not destroy postgres buffers.
I agree your problem is most related to dirty background ration, where buffers are READ only and have nothing to do with disk writes.

From: strahinjak(at)nordeus(dot)com
Date: Thu, 7 Feb 2013 13:06:53 +0100
Subject: Re: [PERFORM] postgresql.conf recommendations
To: kgrittn(at)ymail(dot)com
CC: johnnydtan(at)gmail(dot)com; ac(at)hsk(dot)hk; jkrupka(at)gmail(dot)com; alex(at)paperlesspost(dot)com; pgsql-performance(at)postgresql(dot)org

As others suggested having shared_buffers = 48GB is to large. You should never need to go above 8GB. I have a similar server and mine has

shared_buffers = 8GB

checkpoint_completion_target = 0.9

This looks like a problem of dirty memory being flushed to the disk. You should set your monitoring to monitor dirty memory from /proc/meminfo and check if it has any correlation with the slowdowns. Also vm.dirty_background_bytes should always be a fraction of vm.dirty_bytes, since when there is more than vm.dirty_bytes bytes dirty it will stop all writing to the disk until it flushes everything, while when it reaches the vm.dirty_background_bytes it will slowly start flushing those pages to the disk. As far as I remember vm.dirty_bytes should be configured to be a little less than the cache size of your RAID controller, while vm.dirty_background_bytes should be 4 times smaller.

Strahinja Kustudić | System Engineer | Nordeus

On Wed, Feb 6, 2013 at 10:12 PM, Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:

Johnny Tan <johnnydtan(at)gmail(dot)com> wrote:

> Wouldn't this be controlled by our checkpoint settings, though?

Spread checkpoints made the issue less severe, but on servers with

a lot of RAM I've had to make the above changes (or even go lower

with shared_buffers) to prevent a burst of writes from overwhelming

the RAID controllers battery-backed cache. There may be other

things which could cause these symptoms, so I'm not certain that

this will help; but I have seen this as the cause and seen the

suggested changes help.

-Kevin

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