Message-ID: <480F1955.7020300@lelarge.info>
Date: Wed, 23 Apr 2008 13:11:17 +0200
From: Guillaume Lelarge <guillaume@lelarge.info>
User-Agent: Thunderbird 2.0.0.12 (X11/20080227)
MIME-Version: 1.0
To: =?ISO-8859-2?Q?Gerd_K=F6nig?= <koenig@transporeon.com>
CC: pgsql-admin@postgresql.org
Subject: Re: confused of buffers and memory settings
References: <480F0C92.8050907@transporeon.com>
In-Reply-To: <480F0C92.8050907@transporeon.com>
Content-Type: text/plain; charset=ISO-8859-2; format=flowed
Content-Transfer-Encoding: 8bit

Hi,

Gerd König a écrit :
> we're quite new to Postgres and we're going to start using Postgres8.3 
> on Opensuse10.3 64 bit. We have a dedicated dbserver with 8GB RAM, and 
> now I'm not sure how to deal with the memory/buffer settings. The db 
> will be accessed heavily (~20 requests/sec.) with a read/write ration of 
> 50:50, yes, a lot of write activity.
> 
> I thought of setting "shared_buffers" to 750000 (~6GB) but how depends 
> this on the kernel buffer setting in /etc/sysctl.conf (what is the 
> interaction between these two settings?).
> I know the variable "shmmax" can be defined, but currently there's no 
> such entry.
> 

shared_buffers should be set to 1/4 of your available RAM. It's a start 
value, you can tweak it after that, but you would probably be better 
advised to use 2GB to begin with.

> The meaning of "work_mem" / "maintenance_work_mem" and "wal_buffers" is 
> also not clear. The maintenance_work_mem influences the size of the WAL 
> logs..?!?
> 

work_mem is used for sort and group operations (for example, ORDER BY). 
It's not part of the shared_buffers memory and it will be used by every 
postgres process. So you better have small values, something between 1 
MB and 64 MB.

maintenance_work_mem is used for maintenance operations (VACUUM, CREATE 
INDEX and another one I don't remember now). You can use bigger values 
because you won't have many operations of this kind that will happen at 
the same time. Probably 256 MB would be a good start value.

> What else are "top performance related" options for the usage scenario I 
> described earlier ?
> 

Checkpoint and WAL settings are important ones. FSM size is another one.

Regards.


-- 
Guillaume.
  http://www.postgresqlfr.org
  http://dalibo.com