Re: Cache Hit Ratio from system views

On Aug 28, 4:45 pm, "Bob Jones" <em..._at_me.not> wrote:
> <hjr.pyth..._at_gmail.com> wrote in message
>
> news:1187675622.406144.322710_at_x35g2000prf.googlegroups.com...
>
>
>
>
>
> > On Aug 21, 3:19 pm, "Bob Jones" <em..._at_me.not> wrote:
> >> "Richard Foote" <richard.fo..._at_nospam.bigpond.com> wrote in message
>
> >>news:fgixi.22091$4A1.5979_at_news-server.bigpond.net.au...
>
> >> > "Bob Jones" <em..._at_me.not> wrote in message
> >> >news:eB8xi.1326$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.
>
> > How about you first dealing with some of the issues that have been
> > raised?
>
> > THIS indicator is not worth caring about because its meaning is
> > ambiguous and therefore it is non-prescriptive: it cannot tell you
> > what to do to improve a problem, let alone whether you have a problem.
>
> > OTHER indicators, however, are not so ambiguous. A low parse/execute
> > ratio would indicate poorly-shareable SQL: use bind variables or
> > switch on CURSOR_SHARING. A high count of RELOADS in v$librarycache
> > would indicate an insufficiently-sized shared pool: increase it. And
> > so on (and yes I'm over-simplifying the indicators and what they
> > indicate for the purposes of this post).
>
> I hope you are not telling me these 2 indicators cannot be inflated by the
> applications.
> What does RELOADS tell me about wether the system is doing useful work?
> What number of RELOADS is good or bad? I have 30000 reloads. Do I need to
> increase my librarycache?- Hide quoted text -
>
> - Show quoted text -

And again you post useless 'examples' with no context. The reloads value can tell much in terms of how the cache is functioning, over a given period of time, with a given user load, with an observed rate of transactional activity. Your 'example' is devoid of such information, but your entire 'argument' in support of the BCHR has been devoid of any supporting data since you decided to start it. I suppose in your mind you can see clearly what you fail to prove to the rest of humanity. It's a shame you can't actually prove this to the rest of the known world.

David Fitzjarrell Received on Thu Aug 30 2007 - 13:06:32 CDT