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

From: Tanel Poder <tanel.poder.003_at_mail.ee>
Date: Thu, 20 Nov 2003 07:24:59 -0800
Message-ID: <F001.005D7447.20031120072459@fatcity.com>


:)

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: Tanel Poder
  INET: tanel.poder.003_at_mail.ee

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Received on Thu Nov 20 2003 - 09:24:59 CST

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