Re: Re: Re: _wait_for_sync , dirty buffer flushing and direct reads in parallel

Hi!

Yup, I was bold enough to use this parameter during production upgrade only because it worked well in several tests and simulations.

Cheers,
Tanel.

> Well,
>
> some disk writes need to wait for the LGWR to flush the corresponding
> redo
> to disk. So now you can have a situation that the blocks that are
> dirty are
> on disk (without a commited transaction) but the redo is not yet. So
> if you
> crash in that period, you can't recover.
>
> Anjo.
>
> ----- Original Message -----
> To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
> Sent: Thursday, November 20, 2003 2:59 PM
> parallel
>
>
> > Anjo,
> >
> > I also thought it affects only lgwr sync, but Jonathan Lewis once
> told
> that it affects any disk writes...
> >
> > If it affects only lgwr, then great, I can make Apps upgrades,
> which do
> really lots of DDLs and small transactions, quite much faster that
> way...
> >
> > Thank you,
> > Tanel.
> >
> >
> > > _wait_for_sync basically meant that a session is waiting for the
> sync
> > > of the
> > > redo by the lgwr. Normally the redo log writer writes to disk and
> then
> > > notifies the session that the transaction is completed. By setting
> > > this to
> > > false, you no longer wait for the redo to go to disk.
> > >
> > > That has no impact on your situation.
> > >
> > > Anjo.
> > >
> > > ----- Original Message -----
> > > To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com>
> > > Sent: Wednesday, November 19, 2003 11:20 PM
> > > query
> > >
> > >
> > > > Hi!
> > > >
> > > > I've sometimes used setting _wait_for_syncúlse during Apps
> upgrade
> > > > projects, to upgrade performance. (As long as your database
> doesn't
> > > crash
> > > > during the parameter is set to false, no problems should occur).
> > > >
> > > > I just started wondering, what would be the case if a parallel
> query
> > > starts
> > > > during someone is modifying data...
> > > >
> > > > As I understand, when doing parallel query:
> > > > 1) the dirty blocks which are supposed to be read by PQ in
> direct
> > > mode,
> > > are
> > > > flushed to disk
> > > > 2) PQ reads the blocks in direct mode
> > > >
> > > > But when _wait_for_sync is set, the writes get acknowledged
> > > immediately
> > > (or
> > > > acknowledgement is not waited for). Could this result in the
> > > unlikely
> > > > situation, that PQ issues the flush command to dirty buffers and
> > > starts to
> > > > read them, but actually reads the old images of the blocks,
> since it
> > > thinks
> > > > the write has already occurred?
> > > >
> > > > (actually, this doesn't touch only PQ, it's possible to have
> direct
> > > reads
> > > to
> > > > PGA in serial mode too...)
> > > >
> > > > Tanel.
> > > >
> > > >
> > > > --
> > > > Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > > > --
> > > > Author: Tanel Poder
> > > > INET: tanel.poder.003_at_mail.ee
> > > >
> > > > Fat City Network Services -- 858-538-5051
> http://www.fatcity.com
> > > > San Diego, California -- Mailing list and web hosting
> > > services
> > > >
> > >
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> > > --
> > > Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > > --
> > > Author: Anjo Kolk
> > > INET: anjo_at_oraperf.com
> > >
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> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Anjo Kolk
> INET: anjo_at_oraperf.com
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Tanel Poder
  INET: tanel.poder.003_at_mail.ee

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