RE: Slow database, too MANY buffers???

I got to see over 1,000 on a couple of cache buffers chains at one Oracle7 site a few years ago. This was prior to when Server Technologies set the limit of CR blocks in the buffer cache to 42 (spooky), and then eventually created (I think) the _db_block_max_cr_dba parameter.

The cause was an application that used a "select for update / update / commit" on one-row/one-column tables to generate sequence numbers
(instead of using real Oracle sequences).

Cary Millsap
Hotsos Enterprises, Ltd.
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-----Original Message-----
Lewis
Sent: Thursday, January 23, 2003 1:50 PM To: Multiple recipients of list ORACLE-L

You could try:

    select file#, dbablk, count(*)
    from x$bh
    group by

        file#, dbablk
    having count(*) > 5
    ;

(technically you should include the

tablespace number, but that won't
matter if you have less than 1022
files).

This will report the blocks which have an unusually large number of CR copies in the buffer. There is a nominal limit of 7, but if your buffer is excessive for the work done then there is a fair chance that the most intensively used blocks will have far more buffers. (The worst case I saw was something like 75).

The side effects of this would include lots of spinning and sleeping on the cache buffers chains latch.

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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-----Original Message-----
To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com> Date: 23 January 2003 19:05

I've got a cust that is showing some signs that one would think is having a horrible problem on I/O...

I suspect it is the "error" of having a **HUGE** value in db_block_buffers and it's constantly crunching memory trying to figure out which ones to free up. It's only 256 meg, but depending on load, it may be too much. SGA is 687 meg.

While I've got hundreds of SQL, I'm not sure I have one to diagnose buffer utilization. It's also version 8.1.7 and I could probably do some tweaking of buffers.

It's Oracle CRM with a lot of customization and I'm also finding some SQL that's getting a bit ugly under there.

Maks.

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  INET: jonathan_at_jlcomp.demon.co.uk

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-- 
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Author: Cary Millsap
  INET: cary.millsap_at_hotsos.com

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