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Re: Read/Write block sizes (Was: Caching by Postgres)

From: "Jeffrey W(dot) Baker" <jwbaker(at)acm(dot)org>
To: Michael Stone <mstone+postgres(at)mathom(dot)us>
Cc: Jignesh Shah <J(dot)K(dot)Shah(at)Sun(dot)COM>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Read/Write block sizes (Was: Caching by Postgres)
Date: 2005-08-23 23:44:10
Message-ID: 1124840650.12932.1.camel@toonses.gghcwest.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, 2005-08-23 at 19:12 -0400, Michael Stone wrote:
> On Tue, Aug 23, 2005 at 05:29:01PM -0400, Jignesh Shah wrote:
> >Actually some of that readaheads,etc  the OS does  already if it does
> >some sort of throttling/clubbing of reads/writes.
> 
> Note that I specified the fully cached case--even with the workload in
> RAM the system still has to process a heck of a lot of read calls.
> 
> >* Introduce a multiblock or extent tunable variable where you can
> >define a multiple of 8K (or BlockSize tuneable) to read a bigger chunk
> >and store it in the bufferpool.. (Maybe writes too) (Most devices now
> >support upto 1MB chunks for reads and writes)
> 
> Yeah. The problem with relying on OS readahead is that the OS doesn't
> know whether you're doing a sequential scan or an index scan; if you
> have the OS agressively readahead you'll kill your seek performance.
> OTOH, if you don't do readaheads you'll kill your sequential scan
> performance. At the app level you know which makes sense for each
> operation.

This is why we have MADVISE_RANDOM and MADVISE_SEQUENTIAL.

-jwb

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