Hi, Carel-Jan and Rich,
Connor's script to bump up buffer cache hit ratios is meant to be a humor. Only
if you carefully comtemplate it will you see that there's no relevance of the
fact that you can get any hit ratio to the fact that hit ratios are
insufficient in performance tuning.
It would be equally easy to write scripts to bump up some wait event times. If
you need very long db file reads, create a big table and keep scanning it. If
you need long enqueue waits, create a table and insert a row. Create 10 or 100
sessions (depending on your patience) and delete from that table and wait. The
fact that you can get arbitary wait times does not reduce the efficacy of wait
event interface as a performance tuning tool.
Buffer cache or library cache hit ratios are not sufficient, very insufficient
used alone, to tune the database. The reason is that they don't contain enough
information to tune the system with. This is the only reason we should not
solely rely on them; in fact, not using them at all doesn't hurt much. The
reason is not that we can get any value we want by playing pranks.
Hit ratios are still used in other performance tuning and not condemned.
Although in UNIX performance tuning one looks at absolute numbers such as scan
rate, CPU usage and netstat output more often, hit ratios in some sar output
are still occasionally used. Most ratios could still be distored by a rogue
user repeatedly doing, say, "find /" for inodes or "find / -exec grep SomeThing
{} \;" for page cache.
In any tuning practice, Oracle or OS, artificially distorting usage patterns
invalidates your numbers even if you're using a well respected tuning method.
So only play pranks on a play box, not production.
Yong Huang
At 11:14 22-12-03 -0800, you wrote:
>My BCHR is currently 96.62%. In the past, it was normally over 99%. What
>should I do?
>
>I'll be waiting for Mladen's reply... :)
>
>
>Rich
>
>Rich Jesse System/Database Administrator
>[EMAIL PROTECTED] Quad/Tech Inc, Sussex, WI USA
Go to www.oracledba.co.uk (Connor) or go to O'Reilly (download page of
Cary's book), and download one of the fabulous BCHR enhancement scripts.
Especially when your bonus depends on it, this is a good time to perform
some BCHR tuning.
Regards, Carel-Jan
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