Re: Cache Hit Ratio from system views

"Brian Peasland" <dba_at_nospam.peasland.net> wrote in message news:46cc3306$0$16330$88260bb3_at_free.teranews.com...
> Richard Foote wrote:
>>
>> "Bob Jones" <email_at_me.not> wrote in message
>> news:kOtyi.50198$YL5.8637_at_newssvr29.news.prodigy.net...
>>>
>>> High BCHR is always better than low - provided everything else being
>>> equal. If BCHR is useless for the stated reasons, no other indicator
>>> would be useful.
>>
>> This I'm afraid is where you're fundamentally incorrect.
>>
>> A high BCHR can mean your database is on life support, struggling to cope
>> with exessive LIOs due to inefficient SQL with users staring at an
>> hourglass rather than returned data.
>>
>> A BCHR that has increased can mean your database has suddenly hit
>> significant performance issues. Or it can mean things have improved. Or
>> it can mean response times remain unaffected.
>>
>
> <<snip>>
>
> This is what I've said for many years now. If you know your instance
> typically shows a BCHR of X% and today it is showing a BCHR of Y% (where Y
> is not in the neighborhood of X), then all you can say is that something
> has changed. You cannot tell what has changed and you cannot tell if
> things are better or worse. If X%>Y%, the only thing you can positively
> state is that X%>Y% and that's it!
>
>

And of course if X still equals Y, something could have changed making performance better or worse as well, for all or some of your users.

Therefore whatever database checks one makes still need to be made regardless of the BCHR.

Again, it's not much of an indicator ...

Cheers

Richard Received on Wed Aug 22 2007 - 16:25:12 CDT