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Re: Database Hit Ratios

From: Richard Foote <>
Date: Tue, 9 Jul 2002 14:16:23 +1000
Message-ID: <UltW8.31033$>

Hi Conner,

I thought some free advertising can't hurt :)

I completely agree.

The 'chuck more resources to fix it', the 'she'll be right mate we'll worry about all that tuning stuff later' mentality is very prevalent at the moment. Resources are (relatively) cheap and DBAs are (relatively) expensive. Problem is of course that resources is not *A* solution, never mind *THE* solution. I've never seen a 100 million row FTS out perform a single row index access not matter the available resources. A skilful Oracle DBA is still a very valuable (and cost effective) commodity.

I just wanted to stimulate some thinking on all this. Thanks heaps for being a part in my being able to do so.



PS. I knew Hewitt could do it. Go you aussies !!

"Connor McDonald" <> wrote in message
> Richard Foote wrote:
> >
> > Hi All,
> >
> > I know I'm sticking my neck out here, but recently many people have been
> > commenting that database hit ratios matter about as much to database
> > as Mike Tyson matters to world peace (ie. not a lot). I've read the
> > excellent articles by Gary Millsap, seen the very clever dial up script
> > Connor McDonald and read many comments here. I'm sure I'm being
> > but (gulp) I disagree with any final conclusion that suggests hit ratios
> > *never* matter. What's important (and this is what I've been failing to
> > hear), is that hit ratios matter IF *interpreted* and used in a logical
> > meaningful manner.
> >
> > Let me position my case, with an emphases on the database buffer cache
> > ratio. Firstly people who tune " SQL statements" based on hit ratios are
> > wrong. People who tune databases to a "specific" hit ratio are wrong.
> > who think a database hit ratio of 99.99 means their database is king
> > are wrong.
> >
> > In fact before you seriously look at memory performance, you *must*
> > the correct (and in my opinion only) tuning methodology. That is first
> > your business requirements accurate and appropriate. Then get you
> > modelling and data design accurate and appropriate. Then implement your
> > database design efficiently and appropriately. Then get your
> > designed efficiently, accurately and appropriately. (Difficult for out
> > the box solutions I know...). The net effect of all
> > this is that you now have a database that has been tuned to do the
> > possible amount of work to satisfy it's existence. This means you have
> > reduced the number of *logical* I/Os down to an absolute minimum (which
> > the end of the day is what 95% of database tuning is all about). Unless
> > has all been effectively performed, then memory tuning is a little like
> > mechanic tuning a car only to have the driver plod away in first gear !!
> >
> > Now we get to the hit ratio and it's useful and valid interpretation
from a
> > tuning perspective. I've seen Conner's "dial up" hit ratio example (and
> > clever it is too) but I view it as an invalid way to discredit the
> > interpretation* of the hit ratio. Invalid in that it runs a little
> > that
> > accesses (as many times as necessary) a cached table that increases the
> > logical reads to the point the physical reads are an inconsequential
> > percentage of the total logical reads. High hit rate, c**p DB, hit rates
> > mean nothing is the conclusion. But as already stated, a good DBA would
> > detected this rather naughty script, it's irrelevance to business
> > requirements and out she goes. Reducing the logical I/Os to the bare
> > is the prerequisite to memory tuning and the *valid interpretation* of
> > hit rate.
> >
> > In fact, it's the *physical* I/Os that are of most importance here in
> > relation to the hit ratio. Once and only once the logical I/Os have been
> > tuned, do we need to determine the *number* of physical reads. If by
> > increasing my memory buffers, I'm reducing the physical I/Os to the
> > where it's a worthwhile return in investment, then I'm altering the *hit
> > ratio* in a valid and deterministic manner. Once the physical reads have
> > plateaued out, then my tuning is done. The *value* of the hit ratio is
of no
> > consequence, but the behaviour pattern of reducing the physical reads
> > thus it's effect on the hit ratio) is important. And Oracle supports
> > (bless them), why else do they go to such trouble as providing us with
> > the db cache advise stuff, so we can effectively tune the buffer cache
> > reduce physical I/Os and yes, (in)directly the hit ratio performance as
> > well.
> >
> > The buffer cache can actually be too big and hurt performance so
> > it's size without any detriment effect on the physical I/O count is also
> > valid tuning outcome. Again, using my interpretation of the hit ratio
> > statistics to positive effect.
> >
> > I now have a database with the minimum number of *logical* I/Os and the
> > minimum number of *physical* I/Os. MY INTERPRETATION OF TUNING BASED ON
> > RATIOS and I go back to what I said at the start. It's not just the
> > statistics that constitute the hit ratio that are very important, but
> > they are interpreted. And when people say that hit ratios matter not,
it's a
> > blanket statement that worries me because it's somewhat in the eye of
> > beholder exactly what that statement means. It's a correct statement
> > many interpretations but not *all*. Yes, a 95.28% hit ratio is
> > but the number of logical and physical I/Os is very very important.
> >
> > To those that I'm stating the bleedin obvious please forgive me, but
it's a
> > subject that has caused much confusion and this is my little attempt to
> > clarify (confuse?) the matter.
> >
> > Ok, my eyes are close. Hit me.
> >
> > Richard


> Oooh - my name's been mentioned - time to jump in :-)

> I've recently given a presentation at some user group meetings here in
> the UK, which I think better explains my position on cache hit ratios.

> My main problem is the "sequencing" (for lack of a better term) which
> I'll try explain thus (with apologies in advance for length):

> When I first got into the IT industry all I did was cobol/vsam and on
> the odd lucky occasion, some DB2. But when we wrote a program that
> didn't run as well as it should have, the sysops would simply chuck it
> out of the job class and abend the thing. There was only one solution -
> you changed the code to make it more efficient. There was no "can we
> get more cpu, can be get more ram" - hell, we didn't even know how much
> resources the mainframe had - it just ran things. The emphasis was
> simple - if your programs run poorly - you need to fix the program.

> Nowadays, (imho) we've forgotten this history - and I'm not just
> referring to cache hit ratios. Problem: The program sorts too much,
> Soln: increase the sort area size. Problem: Too much IO, Soln: bump up
> the buffer cache. Even on a home PC, OS's have that remarkable ability
> to consume all possible resources. It seems that the *last* thing on
> the list of things to do is fix the program, ie, the "sequencing" is all
> wrong.

> Perpetuating that problem was that not only did we start throwing
> resources at problems, we started using how much we threw as a measure
> of improvement! (cache hit ratio being the obvious candidate).

> I think maximising of the *effectiveness* of the cache (buffer, library,
> whatever) is vital to a DBA's skill repetoire - but that's vastly
> different to trying to get your database to have hit ratio of 'x'
> percent.

> Cheers
> Connor

> PS - in terms of DBA finding the rampant "choose_any_hitratio" code, one
> could always wrap it up in DDL to make identification all that much
> harder :-)

> --
> ==============================
> Connor McDonald


> "Some days you're the pigeon, some days you're the statue..."
Received on Mon Jul 08 2002 - 23:16:23 CDT

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