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Re: Redo Log Question

From: Paul Brewer <paul_at_paul.brewers.org.uk>
Date: Wed, 25 Dec 2002 19:29:38 -0000
Message-ID: <3e0a0c00_1@mk-nntp-1.news.uk.worldonline.com>


"Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message news:PNgO9.9706$jM5.27709_at_newsfeeds.bigpond.com...
>
> "Karen Abgarian" <abvk_at_ureach.com> wrote in message
> news:3E09540D.50E8E55C_at_ureach.com...
> > > > So, if the OS can mirror corruption, why wouldn't Oracle mirror it
> > > > too? I have SEEN BOTH (how about you?) the OS AND Oracle mirror
> > > > "software" corruption.
> > >
> > > Because with OS mirroring, you have LGWR only writing once, and then
> that
> > > being copied by the OS to the mirror. One write. One stuff up. One
> > > corruption. Mirrored. With multiplexing, LGWR writes twice (or three
> times,
> > > if you do 3-way multiplexing). With two (or more) writes, it is
highly
> > > unlikely that LGWR would introduce the same corruption at the same
point
> in
> > > the redo stream. Therefore, multiplexing protects you against software
> > > corruption.
> >
> > An interesting problem here. Suppose LGWR introduced some corruption
into
> > one of the group members (actually it will likely be OS, not LGWR
because
> > the latter writes the same information, but it does not matter). The
redo
> log
> > member
> > is archived by the ARCH. What is really to stop it from taking the
faulty
> one?
>
>
> Yawn.
>
> ARCH does not sit there with a keyboard, tapping out 'cp *.log
> /backup/*.arc'.
>
> It *reads* the redo in the online logs, and writes it into a new file
which
> we call an archived log.
>
> Point is: it READS the log. It knows if it contains corruption. If it
> encounters corruption, it skips to the next member. If it encounters
> corruption there, it moves onto the third member and so on.
>
> It is NOT like some idiot junior DBA doing O/S copies.
>
> Therefore, it is NOT a matter of chance, and this isn't like winning the
> lottery.
>
> HJR
>

Idiot Junior DBA here.

As RF said: 'Tis the season to be jolly............

Merry Christmas, Howard and all.

Regards,
Paul Received on Wed Dec 25 2002 - 13:29:38 CST

Original text of this message

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