Re:os cache vs. db cache
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 10 Jul 2007 17:50:20 +0100
Robyn,
Since you're running a data warehouse, you may want to consider
the effects of parallel tablescans and parallel index fast full scans,
which do direct path reads and bypass the buffer cache.
Of course, there are parallel execution paths that still USE
the buffer cache (parallel nested loop joins with indexed access,
access through partitioned indexes etc.) but if a very large
percentage of the work you do is bypassing the Oracle cache,
you may want to switch memory to the file system cache instead
to try and keep some important objects "close to" the action.
Alternatively, you might think about putting the temporary tablespace
onto a filesystem that has caching enabled so that blocks written to the
temp tablespace are in the cache (and not on disc) when you start to
re-read them.
Yet another option is to keep the caching low - depending on a SAN
cache (for exanple) to help you with the TEMP space - and make as
much memory as you can available for PGA activity so that you minimise
the number of cases where sorts or hash joins spill to disc.
It's not a clear cut choice, unfortunately - often it's a question of observing
the current activity and making the best guess about how moving memory
around will benefit the critical processes.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
----- Original Message -----
Date: Mon, 9 Jul 2007 14:03:42 -0400
From: Robyn <robyn.sands@xxxxxxxxx>
Subject: os cache vs. db cache
Hello everyone,
Over the years, I've read differing opinions on balancing the os cache and
the database cache. Seems like I remember there were some advocates for
minimizing the os cache and maximizing the database cache to allow Oracle to
better determine which blocks should truly be cached and avoid necessary
blocks being cached because they were in proximity to hot data. I also have
heard opposite opinions.
I'm specifically focused on performance for a data warehouse. Has anyone
tested performance with a minimized OS cache and a maximized database
cache? If so, what were the results? Does anyone have a really good paper
or book on the topic? I'm trying to collect some information before I start
discussions with the platforms guys.
tia ... Robyn
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