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

The foreground process is affected. Instead of waiting for the LGWR, it will return right away.

Anjo.

• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Thursday, November 20, 2003 4:24 PM in parallel

> :)
>
> I admit, that I don't know either, which processes are affected by this
parameter.
> If foreground ones are, that should mean that after posting lgwr, they
won't wait on semaphore and continue their work.
>
> If it affects lgwr, it means that lgwr posts the waiting processes
immediately "back" before writing to disk.
>
> I'm too lazy now, but it can probably be figured out by tracing semop()
syscalls... or maybe Steve Adams happens to read this post :)
>
> About "scientific" test results, the results I posted earlier, they were
done exactly on same hardware, with same dataset and from same starting point. Enough scientific for me.
>
> Tanel.
>
> > I think my understanding was wrong. _wait_for_sync actually only
> > changes the
> > behavior of foreground processes. When set to false, they don't wait
> > for
> > LGWR
> > to write redo records to disk; instead they continue to do their work
> > as if
> > log
> > file sync already finished. It *does not* change any behavior of LGWR,
> > notification or not. Correct me if I'm wrong again.
> >
> > I'm still interested in Tanel's benchmark, though. Only that is
> > scientific.
> >
> > Yong Huang
> >
> > --- Yong Huang <yong321_at_yahoo.com>, i.e. myself, wrote a few minutes
> > ago:
> > > Tanel,
> > >
> > > Did you observe better performance? By how much? Do please let us
> > know!
> > >
> > > From what I read, _wait_for_sync when set to false means LGWR
> > immediately
> > > notifies user (foreground) processes that redo record writes are
> > done
> > (even
> > > though they're not). When you say the parameter only affects LGWR,
> > you
> > need
> > > to
> > > clarify what you mean by "affect"; it changes the notification
> > (posting)
> > > behavior of LGWR therefore changes the behavior of waiting processes
> > (*when*
> > > they stop waiting). Just semantics.
> > >
> > > Yong Huang
> > >
> > > --- Tanel Poder <tanel.poder.003_at_mail.ee> wrote:
> > > > 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.
> >
> >
> > __________________________________
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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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