Re: Data Buffer Cache

God knows what button I pressed, but I was in mid-flow, so I'll continue...

"Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message >
> >You then have less issues
> > with redo log sizes,
>
> Principles dictate otherwise. You need to establish a "regular heartbeat"
> before you start monkeying with the rythym.
>
> >less likelihood of checkpoint not completing issues,
>
> That's a function of the number of logs in principle. It can be dealt
with,
> principally, without recourse to extra logs.
>
> >no
> > periodic performance hits
>
> Agreed. Just crap performance all the time instead.
>
> >and predicable instance recovery times.
>
> You make it sound like a menu you can pick and choose. It isn't. It's a
> trade-off. You want predictable instance recovery times? Fine: choose bad
> performance. You want performance? Fine: forget instance recovery times,
> 'cos they're gonna be bad. Strike a balance: fine, but performance will be
> sub-optimal.
>
> And then you have to ask: how often do you expect instance failures? You
> tune for the rare occurance
>

...and that's fine if that's what you're legally required to do. But it strikes me as plain daft to worry about the once-a-blue-moon, and to cripple daily performance accordingly.
>
> The
> > penalty, DBWR working a little harder all of the time,

You make it sound so trivial!

And what of DBWR's 'little' flushes to disk?

Well every damn jack of them conflicts with your user's attempts to read *off* disk, for a start.

Checkpoints are *baaaaaaaaaaaad*. Avoid them where you can. Other things being equal, and lawyers not in attendance. Otherwise, fine: stuff up your database and make it behave like Access on a bad hair day. So long as you know the trade-offs, I trust most DBAs to be able to balance themselves somewhere along the continuum.

But the bed-rock principle remains, whatever, that checkpoints are bad news, and you'd be daft to introduce them where they're not needed.

Regards
HJR
>>how 'little' or how
> > 'much' controlled by the DBA with the tuning of these parameters.
> >
> > Cheers
> >
> > Richard
> >
> >
> > "Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message
> > news:3d87fd79_at_dnews.tpgi.com.au...
> > > This is indeed true, provided you're prepared to undergo continuous
> > > checkpointing. If you have an SLA demanding signed, sealed and
delivered
> > > instance recovery times, fair enough. But otherwise, *anything* that
> > induces
> > > checkpointing when its not needed has got to be a dubious idea at
best.
> > >
> > > I tend to suggest making sure these parameters are either not set, or
> set
> > to
> > > ridiculously high levels so they don't have any practical effect.
> > >
> > > Unless you need the guaranteed recovery times, of course.
> > >
> > > Regards
> > > HJR
> > >
> > >
> > > "Richard Foote" <richard.foote_at_bigpond.com> wrote in message
> > > news:QyPh9.35013$g9.98743_at_newsfeeds.bigpond.com...
> > > > Hi Howard and all,
> > > >
> > > > I think I'm missing something here or maybe it's somewhat out of the
> > scope
> > > > of this discussion (although I don't think it is).
> > > >
> > > > This concept of sizing redo logs in order to control the behaviour
of
> > > > checkpointing. I have no problem with it pre 8i. However it's all
> > somewhat
> > > > irrelevant (or it should be) since the changes in behaviour of the
> > buffer
> > > > cache and the introduction of the fast_start_io_target parameter in
8i
> > > (and
> > > > fast_start_mttr_target in 9i).
> > > >
> > > > By setting these parameters appropriately, the sizing of redo logs
in
> > > order
> > > > to control checkpoint behaviour is no longer an issue per se. Oracle
> > will
> > > > continually post the DBWR to ensure that dirty blocks preventing
these
> > > > targets from being met are flushed to disk.
> > > >
> > > > The advantages of course being *predicable* instance recovery times
> > > > regardless of size of redo logs or when the last checkpoint may have
> > > > completed and an *even* load at all times, no longer there being
> spikes
> > of
> > > > activity as Oracle desperately tries to complete a checkpoint.
> > > >
> > > > I just think it's a point worth mentioning ...
> > > >
> > > > Cheers
> > > >
> > > > Richard
> > > > "Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message
> > > > news:3d878a16_at_dnews.tpgi.com.au...
> > > > > I also tell them of the database in Queensland that produces a
500M
> > > > archive
> > > > > every 7 minutes.
> > > > >
> > > > > I flatter myself that people don't come out of the classroom until
> > > they've
> > > > > got as a complete a picture as it's possible to paint in the time
> > > allowed.
> > > > > If you have to switch that frequently, so be it. Just be aware of
> the
> > > > costs
> > > > > involved. And if you can avoid switching that frequently, it's
> > generally
> > > a
> > > > > good idea to do so, bearing in mind the further potential costs in
> > > > instance
> > > > > recovery scenarios.
> > > > >
> > > > > The bottom line I give them is: size your logs so that you end up
> > > > switching
> > > > > (and hence checkpointing) at a rate you are happy with.
> > > > >
> > > > > Regards
> > > > > HJR
> > > > >
> > > > > "Sybrand Bakker" <gooiditweg_at_sybrandb.demon.nl> wrote in message
> > > > > news:t2oeouk7kk8kugc5m2b3c4vlij4jipr61a_at_4ax.com...
> > > > > > On Tue, 17 Sep 2002 19:12:01 +1000, "Howard J. Rogers"
> > > > > > <howardjr2000_at_yahoo.com.au> wrote:
> > > > > >
> > > > > > >I also tell them of the 'one switch per hour' school of DBAing,
> so
> > > they
> > > > > get
> > > > > > >both sides.
> > > > > >
> > > > > > So what if you often have 250M redolog in less than 30 minutes?
> > > > > > (I'm not joking)
> > > > > >
> > > > > >
> > > > > > Regards
> > > > > >
> > > > > >
> > > > > > Sybrand Bakker, Senior Oracle DBA
> > > > > >
> > > > > > To reply remove -verwijderdit from my e-mail address
> > > > >
> > > > >
> > > >
> > > >
> > >
> > >
> >
> >
>
>
Received on Wed Sep 18 2002 - 04:25:49 CDT