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Re: Cache Hit Ratio from system views

From: Bob Jones <email_at_me.not>
Date: Thu, 30 Aug 2007 02:21:55 GMT
Message-ID: <7RpBi.51274$YL5.45086@newssvr29.news.prodigy.net>

"Richard Foote" <richard.foote_at_nospam.bigpond.com> wrote in message news:cXdBi.27614$4A1.19426_at_news-server.bigpond.net.au...
> "Bob Jones" <email_at_me.not> wrote in message
> news:7d_Ai.4071$JD.3351_at_newssvr21.news.prodigy.net...

>>
>> "Richard Foote" <richard.foote_at_nospam.bigpond.com> wrote in message 
>> news:seAAi.26732$4A1.22707_at_news-server.bigpond.net.au...
>>> "Bob Jones" <email_at_me.not> wrote in message 
>>> news:mOnAi.236$ZA5.16_at_nlpi068.nbdc.sbc.com...
>>>>
>>>> "Richard Foote" <richard.foote_at_nospam.bigpond.com> wrote in message 
>>>> news:2_Wyi.24466$4A1.1328_at_news-server.bigpond.net.au...
>>>>>
>>>>> "Bob Jones" <email_at_me.not> wrote in message 
>>>>> news:kOtyi.50198$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.
>>>>>
>>>>> 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
Received on Wed Aug 29 2007 - 21:21:55 CDT

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