Обсуждение: confused of buffers and memory settings

Hi,

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.

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..?!?

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

any help appreciated....GERD...

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

On Mittwoch, 23. April 2008 Gerd König wrote:
> The db will be accessed heavily (~20 requests/sec.)
> with a read/write ration of 50:50, yes, a lot of write activity.

Not really heavy, I would say, but maybe your transactions are very big
and produce a lot of I/O.

> 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 Mem Maximum example:
kernel.shmmax = 950123456
# do not allow memory overcommit to prevent database crashes
vm.overcommit_memory=2

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

http://www.postgresql.org/docs/8.3/interactive/runtime-config-resource.html

mfg zmi
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Le mercredi 23 avril 2008, Gerd König a écrit :
> we're quite new to Postgres and we're going to start using Postgres8.3
[...]
> What else are "top performance related" options for the usage scenario I
> described earlier ?

Did you read this document already?
  http://www.westnet.com/~gsmith/content/postgresql/pg-5minute.htm

--
dim