Re: redo log generation and undo_retention
- From: "Daniel W. Fink" <optimaldba@xxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Thu, 13 May 2004 12:27:00 -0600
Josh,
Undo entries are always written into the redo stream. This is the only
way to reconstruct undo/rollback segments in order to perform the
rollback tasks of recovery. Undo is always 'inserted', never 'updated'
nor 'deleted' (this is a good way to think about it, though it is not
100% accurate). When an existing block is used by a new transaction, the
block is basically renewed. (again, not 100% accurate, but close enough
for 99% of the issues). The difference in undo_retention settings will
be how long Oracle attempts to keep undo entries from being overwritten.
As far as Oracle is concerned, undo entries into a new block or reused
block are pretty much the same.
Daniel Fink
Jos wrote:
>Dear List,
>I am trying to find out if setting undo_retention to non-zero value will
>generate more redo log or not. I dummy up some update and insert transactions
>and ran it in a 9.2 db with undo_retention=0 and undo_retention=900 (the
>default), I monitored the redo size by querying 'redo size' stat in v$sysstat.
>And I found out that the amount of redo generated under both undo_retention is
>about the same. I would have thought that with undo_retention=non-zero value
>will generate more redo because of the undo entries and these undo will be in
>the redo stream as well. However, from the test result it doesn't seem to be
>the case. Is it my understanding of undo is wrong or the test I did is not
>right?
>Josh
>
>
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