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Date: Tue, 22 Jul 2008 14:08:32 +0000 (GMT)
From: Valentin Bogdanov <valiouk@yahoo.co.uk>
Reply-To: valiouk@yahoo.co.uk
Subject: Re: shared_buffers and shmmax
To: posgres support <pgsql-admin@postgresql.org>,
	dx k9 <bitsandbytes88@hotmail.com>
In-Reply-To: <BLU144-W43110EE920E5B083034F47D1850@phx.gbl>
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Message-ID: <818191.79570.qm@web25804.mail.ukl.yahoo.com>

shared_buffers is in disk block size, typically 8K, at least that's what it=
 is on Linux platforms. shmmax is quite simply in bytes.

The default shared_buffer of a 1000 is quite conservative. A good starting =
value is something like 15-25 percent of your main memory or so I am being =
told. It really depends on how the machine you have your database on is bei=
ng used. If postgres is the only application using your box then you can ev=
en set this to 80% of the memory. You're fine as long as postgres does not =
have to resort to using the swap space.

If you set shared_buffers so high that it doesn't agree with your systems s=
hmmax then postgres will give you the required value on startup.


Regards,

Val


--- On Tue, 22/7/08, dx k9 <bitsandbytes88@hotmail.com> wrote:

> From: dx k9 <bitsandbytes88@hotmail.com>
> Subject: [ADMIN] shared_buffers and shmmax
> To: "posgres support" <pgsql-admin@postgresql.org>
> Date: Tuesday, 22 July, 2008, 2:39 PM
> Hi,
> I'm trying to understand what the documentation means
> by bytes per increment, what is the increment supposed to
> be bytes, MB, or Kb.  I have my shared_buffers set to 577
> MB(4 instances) and I'm multiplying by 8400 bytes.  I
> would think I would want to keep everything in bytes and
> not mulitply bytes times MB, but this is what table 17-2
> implies.  If I convert 577 to bytes and multiply, my
> calculator goes exponential on me. I'm going through
> this table and adding up to see what my shmmax should be
> (it's 7.5 GB) out of a total memory of 16 GB with 1000
> max_connections right now.  What should I use as the
> "increment" value in regards to shared buffers,
> 577, 590848 or 605028352 ?=20
> =20
> a) 577 MB (This seems too small)
> b) 590,848 Kb (this seems just right)
> c) 605,028,352 bytes  (this seems too big, I hope it's
> not c)
> =20
> Thanks,
> ~DjK
> =20
> Table 17-2. Configuration parameters affecting
> PostgreSQL's shared memory usage
>=20
>=20
>=20
>=20
>=20
>=20
>=20
> Name
> Approximate multiplier (bytes per increment) as of 8.3
>=20
>=20
> max_connections
> 1800 + 270 * max_locks_per_transaction
>=20
> autovacuum_max_workers
> 1800 + 270 * max_locks_per_transaction
>=20
> max_prepared_transactions
> 770 + 270 * max_locks_per_transaction
>=20
> shared_buffers
> 8400 (assuming 8 kB BLCKSZ)
>=20
> wal_buffers
> 8200 (assuming 8 kB XLOG_BLCKSZ)
>=20
> max_fsm_relations
> 70
>=20
> max_fsm_pages
> 6
>=20
> Fixed space requirements
> 770 kB
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