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

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
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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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