Re: Redo Log Question

"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 Received on Wed Dec 25 2002 - 05:49:23 CST