Thursday, March 15, 2007

Ratios redux

I have got myself embroiled with Don Burleson in another OTN forum thread on cache hit ratios. In it he asks me
'Surely, you don;t(sic) mean that you would dismiss something because it's "not always relevant"?

If it applied on only 5% of the cases, it's still useful then, right?'


I think we can quibble for ever over the usefulness of a metric that is not applicable in 95% of cases. Of course we can use our skill and judgement to evaluate the importance of a low cache hit ratio is meaningful. But how much investigation is required to determine whether it matters? Is that the best use of our time? In most tuning situations I come across these days it doesn't seem relevant. YMMV (Don's obviously does).

Funnily enough one of the other threads I mentioned in my previous article demonstrated a situation where a low cache hit ratio was indeed an indicator of a bad situation. When the OP finally posted the relevant init.ora parameters we discovered

buffer cache size = 32 mb
shared pool = 76 mb

As Laurent Schneider observed, that's barely enough memory for a pocket calculator.

9 Comments:

Blogger Laurent Schneider said...

I had a good laugh on the script from 3360 ! Quite useful for to fulfill your SLA ;-)

15 March 2007 13:55:00 GMT-7  
Blogger APC said...

Yeah, I like Connor's script too, I've been pointing people to it for years. If you ever get the chance see Connor present do grab it. He is a very entertaining presenter

Cheers, APC

16 March 2007 00:55:00 GMT-7  
Blogger Laurent Schneider said...

ROFL, poor poor poor squirrel

16 March 2007 04:39:00 GMT-7  
Blogger Joel Garry said...

This is why the first thing I do on a new-to-me system is just poke about and see if it looks like some newbie accepted all the defaults or something. I can't see why people throw all sorts of advice like use method-r or bhcp or statspacks at someone who hasn't even got the system to a reasonable starting point. You ought to be clued in when someone doesn't know how to show or change init.ora parameters... talk about low-hanging fruit...

word: gfewz

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I think the comment above is a spam

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