buffer cache hit ratio. LIO vs PIO

[DaPi]

"all generalisations are false"

"there are lies, damn lies and statistics" - a percentage is a statistic, and therefore to be treated with care.

If we just condsider the "work" to do I/O (which is not all the story) then there is a measure of work that will be something like:
Work = A*LIO + B*PIO
where B > A (or even >> A).

If the missRatio is PIO/LIO then:
Work = A*LIO*(1 + C*missRatio)
where C > 1 (or even >>1).

For a fixed amount of LIO, yes it is good to reduce the missRatio (perhaps by increasing the cache size).

The missRatio is PIO/LIO - it is usually NOT a good idea to reduce this by increasing LIO (though in rare cases it can reduce PIO). Millsap's point is that small missRatios can be the symptom of large LIO values.

I seem to remember Tom Kyte saying "look after the LIO and the PIO will look after itself" - another generalisation, but probably a good starting point.

My experience is that the Hit Ratio is a way to quickly check that all is well in your db once you have understood what is going on in your db.