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Re: Cache Hit Ratio from system views

From: Bob Jones <email_at_me.not>
Date: Fri, 31 Aug 2007 03:49:30 GMT
Message-ID: <edMBi.4488$JD.4477@newssvr21.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.
>>>>>>>
>>>>>>> A BCHR that has reduced can mean your database has suddenly hit
>>>>>>> significant performance issues. Or it can mean things have improved
>>>>>>> (yes, improved because that crippling transaction that was
>>>>>>> previously performing poorly due to massively exessive LIOs has been
>>>>>>> fixed, reducing the overall BCHR) . Or it can mean response times
>>>>>>> remain unaffected.
>>>>>>>
>>>>>>> Not much of an indicator is it ?
>>>>>>>
>>>>>>> But saying that a BCHR is *always* better than a low is just plain
>>>>>>> wrong wrong wrong ...
>>>>>>>
>>>>>>
>>>>>> Didn't I repeatedly say "provided everything else being equal"?
>>>>>>
>>>>>
>>>>> And how precisely do you determine that everything else indeed is
>>>>> equal ? Most databases don't exactly remain equal ...
>>>>>

>>>>
>>>> No, they do not. That's why you do not look at BCHR alone, as I have 
>>>> said before.
>>>
>>> So what else do you look at in conjunction with the BCHR ?
>>>
>>> Interestingly, you never answer any of the questions and you never give 
>>> any examples of why you consider the BCHR to be such a fantastic 
>>> indicator. And yes, I have read *all* your contributions to this 
>>> discussion ...
>>>
>>> So how about you at least attempt to justify your claim that the BCHR is 
>>> "a very meaningful indicator". How do you actually use the BCHR in a 
>>> meaningful manner ? So you look at the BCHR and ..., and what ?
>>>
>>> And when do you look at these other "whatevers" in conjunction with the 
>>> BCHR ? When the BCHR increases, what else do you check ? And when the 
>>> BCHR decreases, what else do you check and how do these checks differ 
>>> from when the BCHR increases ? And when the BCHR remains the same, what 
>>> else do you check and how do these checks differ from when the BCHR 
>>> increases or decreases ?
>>>
>>> Remember, it's your claim that the BCHR is "a very meaningful 
>>> indicator", well show us ?
>>>
>>> If you can ....
>>>
>>>>

>>>>> And when precisely do you check if everything else is equal with this
>>>>> "very meaningful indicator" of yours ? When the BCHR increases ? When
>>>>> the BCHR decreases ? When the BCHR remains the same ?
>>>>>
>>>>
>>>> Try asking yourself the same questions about any other indicators you 
>>>> consider meaningful. The question here is not how to determine if 
>>>> everything else is equal. It is about whether BCHR means anything if 
>>>> everything else is equal.
>>>
>>> Please, if everything else is equal, how can the BCHR change ? How can a 
>>> high BCHR always be better than a low BCHR, everything being equal when 
>>> having a higher BCHR can only mean things are not equal by definition, 
>>> else the BCHR would be the same ? Right ?
>>>
>>> Can you please explain how this is possible, having a higher BCHR with 
>>> everything being equal, at least attempt some kinda description of what 
>>> "everything else" means, at least attempt to justify this somewhat 
>>> bizarre claim ...
>>>
>>> If you can ...
>>>
>>> Again I go back to my initial set of questions. If your BCHR were to 
>>> increase from (say) 95% to (say) 99.9%, if this very meaningful 
>>> indicator were to change in this manner, what else do you check to 
>>> ensure that things are really better, that the higher BCHR is actually a 
>>> good thing, that all these mysterious "things" are indeed equal ?
>>>
>>> And why wouldn't you need to check these other indicators when the BCHR 
>>> decreases ?
>>>
>>> And why wouldn't you need to check these things if the BCHR remains the 
>>> same ?
>>>
>>> If you can't answer these rather basic questions is a vaguely meaningful 
>>> manner, then ummmm, game over I think.
>>>
>>> Go on, answer these questions, dare ya !!
>>>
>>> If you can ...
>>>
>>> Cheers
>>>
>>> Richard
>>
>
> Hey Bob, your best contribution to this whole discussion to date ...
>
> Cheers
>
> Richard

Coming from someone who can't read, I am not exactly flattered. Received on Thu Aug 30 2007 - 22:49:30 CDT

Original text of this message

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