Re: Data Buffer Cache

Hi Howard,

Cool !! (I knew we'll get there ;)

The other issue of note which is applicable in the example I gave is that they have a standby database, which requires 'regularish' (is that a word ?) log switches (8i).

If you have no log switches and no checkpointing (except at midnight) then yep, you've got me, that's about as clear of checkpoints as you can get. I must say though that I've never come across an organisation that has the pendulum swung quite that far in favour of performance vs. instance recoveries (then again, I've lived a sheltered life here in sunny Canberra).

I personally favour and recommend a more balanced approach through incremental checkpointing but now I see where you're coming from.

Thanks for the discussion (although I pity any poor sod trying follow it ;)

Cheers

Richard

"Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message news:9gqi9.36098$g9.102666_at_newsfeeds.bigpond.com...
> (Posted once before, but it seems not to have arrived, so here it is
again,
> with a couple of minor changes).
>
> "Richard Foote" <richard.foote_at_bigpond.com> wrote in message
> news:kwYh9.35307$g9.100157_at_newsfeeds.bigpond.com...
> >
> >
> > That is an incorrect assumption. Performance can actually *improve* as
the
> > expensive checkpoints that did impact performance are gone, and an
> > unaggressive recovery target can be applied that doesn't actually do
much
> > additional work as the requested blocks could very well be long gone
from
> > the buffer cache.
> >
> > *This is a very key point * ...
> >
>
> Richard Foote" <richard.foote_at_bigpond.com> wrote in message
> news:kwYh9.35307$g9.100157_at_newsfeeds.bigpond.com...
> >
> > That is an incorrect assumption. Performance can actually *improve* as
the
> > expensive checkpoints that did impact performance are gone, and an
> > unaggressive recovery target can be applied that doesn't actually do
much
> > additional work as the requested blocks could very well be long gone
from
> > the buffer cache.
> >
> > *This is a very key point * ...
> >
>
> And one that's missed mine by a mile (which was that I wouldn't have any
> checkpoints at all, so performance *would* suffer in comparison when you
> induce additional checkpointing).
>
> To try and cut through all the junk that this thread's thrown up, here's a
> summary of what I recommend:
>
> No checkpoints at all if you don't need to worry about recovery times (ie,
> huge logs, and all the various extra-checkpoint-inducing
> parameters switched to non-effective settings), and performance is King.
> At midnight, schedule an 'alter system
> checkpoint'. Performance during the day will be the best it can be.
>
> If you do have to worry about instance recovery times, then of course
> incremental checkpointing is to be favoured over big-bang
> checkpointing -though the change in behaviour of log_checkpoint_interval
in
> 8i over 8.0 means that you get that even without the use of the 8i/9i
> _TARGET parameters. Performance will not be as good as it would have been
> without any checkpointing at all, but it won't be periodically hammered to
> death, either. Strike a balance between performance and recovery time by
> setting the parameters carefully to limit the amount of checkpointing to
> that which is strictly necessary to achieve the required balance.
>
> If you are mainly worried about instance recovery times, then set the
> fast_start_io_target parameter together with the log_checkpoint_interval
> parameter (or the MTTR_TARGET one in 9i, which sets both in one go).
> Performance will suffer as compared to suggestion #1, but that's fine,
> because you are constrained to worry about other things, and so its a
> trade-off you are willing to make.
>
> I don't think that any of that is capable of being taken exception to.
Where
> we may disagree is that I would usually favour suggestion #1 over any of
the
> others, because I would prefer to tune for regular activity rather than
> concern myself with an event (instance failure) that hopefully only
happens
> once in a blue moon.
>
> Of course, if you're running on NT, that wouldn't apply to you, because
blue
> moons in NT-land are potentially two-a-penny. :)
>
> Now all your examples and counter-arguments are based on (for example) log
> switching once an hour. Where I think we've been at loggerheads is that I
> wouldn't even switch that frequently unless I had to (ridiculous amounts
of
> redo being produced every 3 minutes, legal issues, SLAs etc). You are
> definitiely in Suggestion #2 & #3-land. Which is not where I want to be,
> given half a chance.
>
> HJR
>
>
Received on Fri Sep 20 2002 - 01:18:04 CDT