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From: Greg Smith <[email protected]>
To: [email protected]
Subject: Re: Volunteer to build a configuration tool
Date: Thu, 21 Jun 2007 03:14:48 -0400 (EDT)
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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On Wed, 20 Jun 2007, Campbell, Lance wrote:

> If everything I said is correct then I agree "Why have 
> effective_cache_size?"  Why not just go down the approach that Oracle 
> has taken and require people to rely more on shared_buffers and the 
> general memory driven approach?  Why rely on the disk caching of the OS?

First off, it may help explain the dynamics here if you know that until 
fairly recent releases, the PostgreSQL shared_buffers cache had some 
performance issues that made it impractical to make it too large.  It 
hasn't been that long that relying more heavily on the Postgres cache was 
technically feasible.  I think the user community at large is still 
assimilating all the implications of that shift, and as such some of the 
territory with making the Postgres memory really large is still being 
mapped out.

There are also still some issues left in that area.  For example, the 
bigger your shared_buffers cache is, the worse the potential is for having 
a checkpoint take a really long time and disrupt operations.  There are OS 
tunables that can help work around that issue; similar ones for the 
PostgreSQL buffer cache won't be available until the 8.3 release.

In addition to all that, there are still several reasons to keep relying 
on the OS cache:

1) The OS cache memory is shared with other applications, so relying on it 
lowers the average memory footprint of PostgreSQL.  The database doesn't 
have to be a pig that constantly eats all the memory up, while still 
utilizing it when necessary.

2) The OS knows a lot more about the disk layout and similar low-level 
details and can do optimizations a platform-independant program like 
Postgres can't assume are available.

3) There are more people working on optimizing the caching algorithms in 
modern operating systems than are coding on this project.  Using that 
sophisticated cache leverages their work.

"The Oracle Way" presumes that you've got such a massive development staff 
that you can solve these problems better yourself than the community at 
large, and then support that solution on every platform.  This is why they 
ended up with solutions like raw partitions, where they just put their own 
filesystem on the disk and figure out how to make that work well 
everywhere.  If you look at trends in this area, at this point the 
underlying operating systems have gotten good enough that tricks like that 
are becoming marginal.  Pushing more work toward the OS is a completely 
viable design choice that strengthens every year.

--
* Greg Smith [email protected] http://www.gregsmith.com Baltimore, MD

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