Date: Fri, 25 Jul 2003 12:26:01 -0400 From: Christopher Weimann <cweimann@k12hq.com> To: Tom Samplonius <tom@sdf.com> Cc: Terry Lambert <tlambert2@mindspring.com> Subject: Re: Tuning for PostGreSQL Database Message-ID: <20030725162601.GA35378@smtp.k12us.com> In-Reply-To: <Pine.BSF.4.05.10307242358580.16986-100000@misery.sdf.com> References: <20030724173910.GA9364@smtp.k12us.com> <Pine.BSF.4.05.10307242358580.16986-100000@misery.sdf.com>
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On Fri 07/25/2003-12:03:32AM -0700, Tom Samplonius wrote: > > Maybe you should continue to worry. PostgreSQL isn't MySQL (or a > typical server application). It reads all database pages into its shared > memory area. It is wasteful for the DBMS and the OS to both cache this > data. You'll want the PostgreSQL shared memory size to be around 75% the > size of RAM (on a dedicated DBMS server). In fact, many commercial DBMS > systems will use raw writes to bypass the OS cache! > I was concerned about the disk cache because of this link http://www.varlena.com/varlena/GeneralBits/Tidbits/perf.html#shbuf which says... PostgreSQL counts a lot on the OS to cache data files and hence does not bother with duplicating its file caching effort. The shared buffers parameter assumes that OS is going to cache a lot of files and hence it is generally very low compared with system RAM. Even for a dataset in excess of 20GB, a setting of 128MB may be too much, if you have only 1GB RAM and an aggressive-at-caching OS like Linux. But now that I have looked a bit more I see that this link http://www.postgresql.org/docs/aw_pgsql_book/hw_performance/node6.html which says... Ideally, the POSTGRESQL shared buffer cache will be: * Large enough to hold most commonly-accessed tables * Small enough to avoid swap pagein activity So I have conflicting documentation. I have machine with 4Gig of ram. What is the maximum value of SHMMAX on FreeBSD?
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