Re: redo log generation and undo

From: Daniel W. Fink <optimaldba_at_yahoo.com>
Date: Thu, 13 May 2004 12:27:00 -0600
Message-ID: <40A3BDF4.7060108@yahoo.com>

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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Received on Thu May 13 2004 - 15:35:59 CDT

Re: redo log generation and undo_retention