shared_buffers:
As a reminder: This figure is NOT the total memory PostgreSQL has to work with. It is the block of dedicated memory PostgreSQL uses for active operations, and should be a minority of your total RAM on the machine, since PostgreSQL uses the OS disk cache as well. Unfortunately, the exact amount of shared buffers required is a complex calculation of total RAM, database size, number of connections, and query complexity. Thus it's better to go with some rules of thumb in allocating, and monitor the server (particuarly pg_statio views) to determine adjustments.
On dedicated servers, useful values seem to be between between 8MB and 400MB (between 1000 and 50,000 for 8K page size). Factors which raise the desired shared buffers are larger active portions of the database, large complex queries, large numbers of simultaneous queries, long-running procedures or transactions, more available RAM, and faster/more CPUs. And, of course, other applications on the machine. Contrary to some expectations, allocating much too much shared_buffers can actually lower peformance, due time required for scanning. Here's some examples based on anecdotes and TPC tests on Linux machines:
* Laptop, Celeron processor, 384MB RAM, 25MB database: 12MB/1500
* Athlon server, 1GB RAM, 10GB decision-support database: 120MB/15000
* Quad PIII server, 4GB RAM, 40GB, 150-connection heavy transaction processing database: 240MB/30000
* Quad Xeon server, 8GB RAM, 200GB, 300-connection heavy transaction processing database: 400MB/50000
Please note that increasing shared_buffers, and a few other memory parameters, will require you to modify your operating system's System V memory parameters. See the main PostgreSQL documentation for instructions on this.
--
Shoaib Mir
EnterpriseDB (www.enterprisedb.com)
Thanks,
I've a value of 1000 set for shared_buffers, does this means that I use 8kbX1000=8Mb of Shared Mem?
The definition from the manual is quite confusing:
shared_buffers (integer )
Sets the amount of memory the database server uses for shared memory buffers. The default is typically 32 megabytes (32MB), but may be less if your kernel settings will not support it (as determined during initdb). This setting must be at least 128 kilobytes and at least 16 kilobytes times max_connections.
What does the integer number represent? Number of shared buffers? If yes, what size does each shared buffer have?
"The default is typically 32 megabytes " suggests that this integer could also represent the number of megabytes?!?
In the postgresql.conf file is an ambiguous comment that could induce the idea that each shared buffer has 8 kb.
So, which is the meaning of this integer?
Thanks.
S.
From: Shoaib Mir [mailto:shoaibmir@xxxxxxxxx]
Sent: Monday, April 02, 2007 1:01 PM
To: Sorin N. Ciolofan
Cc: pgsql-general@xxxxxxxxxxxxxx; pgsql-admin@xxxxxxxxxxxxxx
Subject: Re: [ADMIN] Increasing the shared memory
I guess shared_buffers (in postgresql.conf file) will help you here if you have properly setup your kernel.SHMMAX value.
--
Shoaib Mir
EnterpriseDB (www.enterprisedb.com )On 4/2/07, Sorin N. Ciolofan < ciolofan@xxxxxxxxxxxx> wrote:
Hello!
I'd like to ask you if there is any Postgre configuration parameter (like the ones defined in postgresql.conf file) that could be used for increasing the shared memory for Postgre?
Thank you very much
With best regards,
Sorin
