Re: What information is stored in the redo change vectors exactly ?

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Wed, 30 Mar 2005 06:46:13 +0000 (UTC)
Message-ID: <d2dhvl$nlg$1@hercules.btinternet.com>

"Joel Garry" <joel-garry_at_home.com> wrote in message news:1112142645.678478.236540_at_g14g2000cwa.googlegroups.com...
>A slight addition to what Jonathan said:
> http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96566/rcmconc1.htm#476348
> (Detection of Fractured Blocks During Open Backups)
>
> Jonathan: Does this simply mean the vectors in record 2 are larger, or
> does it do something completely different?
>

In this case (tablespace in hot backup mode), there would be four redo records (prior to10g)

Undo Seg header slot acquire
Copy of whole data block before change
Normal undo/change record with 2 vectors
Undo seg header slot clear

A little detail omitted by the manuals, the block copy does not occur on every change to the block, only on the first change after each occasion that the block comes into the buffer. (Including the first change if the block is already in the buffer when the 'begin backup' is issued)

So if a block is in the buffer, and gets changed 100 times, there will be only one full copy.

If the block is in the buffer, and gets changed 100 times, but gets flushed and reloaded 5 times whilst the changes are going on, there will be six full copies.

Regards

Jonathan Lewis

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