Re: Data Buffer Cache

"Richard Foote" <richard.foote_at_bigpond.com> wrote in message news:sNVh9.35181$g9.99519_at_newsfeeds.bigpond.com...
> Hi Howard,
>
> I totally agree that excessive writing of checkpoint blocks should be
> avoided unless you have SLA demanding small instance recovery times.
>
> However, the advantage of continuous checkpointing is that is avoids the
> performance issues associated with the hourly mega checkpoints. Right in
the
> middle of key processing time, we have DBWR burdened with having to flush
> all current dirty blocks to disk. By setting an appropriate
> fast_start_io/mttr_target, you don't have these periodic performance hits.
> The "cost" associated with checkpointing is relatively evenly spread
> throughout.

I don't want to be awkward, but I don't disagree with any of this -but it's a separate issue. Log sizes should be based on the "raw" rate of checkpointing you want to achieve. You want to start introducing extra checkpointing -or even continuous checkpointing- (a) you must have an SLA or (b) you want your head tested.

>
> By selecting a smaller value for these parameters, you make DBWR work more
> aggressively (and yes, you hence potentially suffer the performance hit as
a
> consequence), by increasing these values you make DBWR work less
> aggressively (as the likelihood of the blocks having naturally aged and
been
> written down by DBWR anyway increases).
>
> Note with your recommendation of setting these values sky high,

Now now... I said, either don't set them at all or set them sky high so they have no effect. It depends on whether there's an Oracle default which would apply in the case of total omission. In either case, I mean: make sure they don't apply.

> you
> effectively set continuous checkpointing on. The only difference being
that
> the fast_start_io/mttr_target is unlikely to be the most aggressive target
> used by Oracle and Oracle will instead use the 90% of smallest log file as
> the instance recovery target (unless your log_checkpoint_interval/timeout
> parameters state otherwise). This being the case, it means DBWR will be
> continually posted to check for checkpoint blocks but the likelihood of
such
> blocks still being in memory is low.
>
> And I agree this isn't necessarily a bad strategy. It means no major
> performance hits during a log switch but longish instance recovery and
> shutdown immediate times.
>

Er, run that one past me again? You mean that MMTR_TARGET or FAST_START_IO_TARGET have a bearing on a shutdown immediate??????? Or the

rate of Log switching has a bearing on them??????????????????????????????

You'll have to explain that one, since in my book, a shutdown immediate means "rollback anything not committed" -and whether you've been checkpointing continuously, a thousand times or once has no bearing whatsoever on whether a transaction's been committed.

None of this discussion has a bearing on shutdown immediate times.

> I personally recommend setting these parameters, but in a way such that
> instance recovery times and the potential performance implications are
> balanced according to the database requirements.

This is getting (yet again) a tad tedious. You think I go in to the classroom and say, "Oh to hell with it, set them to anything you like, so long as its a balanced, rational decision"????

"Balance": shmalance.

Principles, deary. Principles.

In *principle* a checkpoint is bad news. Avoid them where you can. Where you can't, fair enough. But be aware of the costs. Then, whatever price you're prepared to pay is a fair one, and reasonable. But be smart and intelligent: that's what DBAs get paid for. So if circumstances dictate, bend the practices. Do what you have to do. But the principles remain, whatever you do.

You decide to implement continuous checkpointing? Fair enough: I'm sure you had good reasons. But the *science* underlying Oracle means X, Y and Z as side effects.

Whether you like the side effects or not is up to you and your "real World" situation, but the science doesn't change one way or another.

>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

The
> penalty, DBWR working a little harder all of the time, 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 - 04:19:36 CDT