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Re: When does LGWR flush the ebuffer?

From: Paul Drake <drak0nian_at_yahoo.com>
Date: 23 Apr 2003 15:56:17 -0700
Message-ID: <1ac7c7b3.0304231456.56fe5746@posting.google.com>


"Richard Foote" <richard.foote_at_bigpond.com> wrote in message news:<Agbpa.20180$1s1.300093_at_newsfeeds.bigpond.com>...
> "Peter" <peter_at_nomorenewsspammin.ca> wrote in message
> news:kur9avoueoqsaaepunact06qcraj7vla7o_at_4ax.com...
> >
> > The LGWR flushes the buffer every 3 seconds, or when there is a
> > commit.
> > Does it ever flush it when the buffer is 1/3 or totally full?
> > If it flushes it when buffer 1/3 full, then it will never get filled
> > up?
> >
> Hi Peter,
>
> The following wakes up LGWR:
>
> - a commit
> - redo log buffer reaching 1/3 full or 1M (whichever comes first)
> - when DBWR wakes up (such as every 3 seconds, checkpoint, etc.)
>
> If you have a bucket with a hole in it which you cover with your finger
> until the bucket is 1/3 full, does that mean the bucket will never get full
> ?
>
> Will that kinda depends on how "turned on" the tap is and how quickly water
> enters the bucket. If water enters at a faster rate than the water leaves
> the bucket via the hole, then yes, eventually the bucket can and will fill
> up.
>
> Same for the redo log buffer. In fact, it's potentially a little worse in
> that until the 1/3 is written out, Oracle can't overwrite that region of the
> buffer. So if your buffer is sized too small, this event can eventuate.
>
> Cheers
>
> Richard

Richard,

What would Bernoulli have to say aabout your analogy?

The rate at which the fluid exits the hole in the bucket is proportional to the square root of the height of the fluid level above it. So the higher the level in the container, the faster the fluid will exit. Also, buckets tend to be wider at the top, hence the rate at which the level increases, decreases given a constant inflow rate.

eventually, whether its over the top, or through the hole in the bottom, the rate of inflow will equal the rate of outflow, and pseudo-equilibrium (steady-state) will be established.

highly non-linear stuff.
formulas provided upon request.

redo log writer wait events are an entirely different matter.

Paul Received on Wed Apr 23 2003 - 17:56:17 CDT

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