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Re: Cache Hit Ratio from system views

From: Richard Foote <richard.foote_at_nospam.bigpond.com>
Date: Mon, 27 Aug 2007 13:44:05 GMT
Message-ID: <FyAAi.26736$4A1.1866@news-server.bigpond.net.au>


"Bob Jones" <email_at_me.not> wrote in message news:eEnAi.234$ZA5.106_at_nlpi068.nbdc.sbc.com...
>
> "Richard Foote" <richard.foote_at_nospam.bigpond.com> wrote in message
> news:OGWyi.24448$4A1.10071_at_news-server.bigpond.net.au...
>> "Bob Jones" <email_at_me.not> wrote in message
>> news:aBuyi.50201$YL5.11519_at_newssvr29.news.prodigy.net...
>>>
>>> "Richard Foote" <richard.foote_at_nospam.bigpond.com> wrote in message
>>> news:fgixi.22091$4A1.5979_at_news-server.bigpond.net.au...
>>>>
>>>> "Bob Jones" <email_at_me.not> wrote in message
>>>> news:eB8xi.1326$i75.244_at_newssvr19.news.prodigy.net...
>>>>>>> Why is BHCR meaningless? The answer should be short and simple. I
>>>>>>> want
>>>>>>> to hear your opinion.
>>>>>>
>>>>>> One can not prove a negative.
>>>>>> Where is your proof BCHR is a reliable indicator of GOOD performance?
>>>>>
>>>>> BCHR alone does not tell you about overall performance. It simply tell
>>>>> you the disk I/O percentage. It is an indicator, a very meaningful
>>>>> one.
>>>>>
>>>>
>>>> If your "disk I/O percentage" is really really high, what does that
>>>> actually indicate ? Does it indicate all is well with the database or
>>>> does it indicate all might not be well ? If you have SQL nasties that
>>>> use index scans inappropriately or incorrectly loop through full scans
>>>> of cached tables again and again and again, you might have users
>>>> experiencing extremely poor response times. Or you might have users
>>>> that are happy with their instant response times. You can't really tell
>>>> and so it doesn't really give you much of an indicator.
>>>>
>>>> If your "disk I/O percentage" is really really low, what does that
>>>> actually indicate ? Does it indicate all is well with the database or
>>>> does it indicate all might not be well ? It might indicate SQL nasties
>>>> that use index scans inappropriately or incorrectly loop through full
>>>> scans of tables (both large or small) and have users experiencing
>>>> extremely poor response times. Or you might have users that are happy
>>>> with their instant response times as all their online transactions run
>>>> instantaneously because the various large batch reports that are
>>>> running and causing all the high "disk I.O percentage" don't directly
>>>> impact them at all. Just the BCHR ...
>>>>
>>>> Sometimes when the BCHR changes from one level to another, it might
>>>> mean there's an issue. Sometimes it doesn't.
>>>>
>>>> The one constant though is that when there are performance issues,
>>>> response times suffer for those folk/processes experiencing the
>>>> performance issues. That can happen if the BCHR is low or high. And the
>>>> actual cause of a performance issue needs to be investigated whether
>>>> the BCHR is high or low to determine an appropriate fix for the issue.
>>>>
>>>> Now if there are performance issues relating to excessive "disk I/O
>>>> percentage" bottlenecks for SQLs that are efficient either in terms of
>>>> LIO counts or execution counts, then an increase in memory might be a
>>>> reasonable cause of action. However, that requires looking at the cause
>>>> of the issue, not the possible symptoms.
>>>>
>>>> Therefore the best indicator, the most meaningful one, is whether
>>>> response times are meeting business requirements or not. And if not why
>>>> not, regardless of the BCHR because a low or high BCHR may or may not
>>>> be contributing to the problem. If response times do meet business
>>>> requirements, then who really cares what the BCHR might be ?
>>>>
>>>
>>> If that's the case, we don't really need to care about any indicator.
>>> Your argument is basically the same as others here. Please read my
>>> earlier postings.
>>
>> Correct, we don't really need to care about any indicator that's as
>> ambigious as the BCHR.
>>
>> However, response times is an idicator that isn't quite so ambigious and
>> hence is something you should care about ...
>>
>
> So you consider repsonse time a metric collected by system? Ok.
> What does 5 seconds response time tell you? What does 5 minutes response
> time tell you?

Are you seriously suggesting having a banking transaction resulting in a customer waiting for 5 minutes doesn't tell you anything about your system ?

Are you seriously suggesting that a BCHR that remains the same is a better and more "meaningful indicator" than a critical business response time that varies from 5 seconds (telling me in answer to your question that application users are happy) to 5 minutes (telling me users are not so happy) ?

Your "very meaningful indicator" hasn't budged at all (still sitting at 99%) but the application has ground to halt ...

You remind me of someone who considered the health and well being of the Titanic to be based on the ratio of notes being played by the string quartet, all things being equal !!

Cheers

Richard Received on Mon Aug 27 2007 - 08:44:05 CDT

Original text of this message

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