Re: redo records

Changes, transaction, are made directly in the datablock (whether directly by a user or from redo during a recovery). A transaction also gets an entry in the rollback segments. A transaction in a data block has a pointer to the rollback segments entries containing info about the transcation. As long as the transaction is not commited then the info in the RBS can't be over written. Once the transaction is commited the info in the RBS is free to be overwritten. Thus when a user comes along and reads a block and sees there is an active transcation they can go look at the RBS to find out if it is commited or not. If there is info in the RBS fine, if not it means the transaction was commited and info in the RBS overwritten.

If a transaction is uncommited it is forcibly in the Rollback Segments. An uncommited transaction can't be overwritten in the rollback segments.

During recovery Oracle apply the redo and then rollback what needs to be from the rollback segments. If after that moment you read a block that has an uncommited transaction, Oracle will check the rollback segments, if the transaction is not there, that means it has been commited. This is called block cleanout, increases efficiency for the original transaction because it doesn't have to mark the transaction as commited, but the user who comes later has to do more work. Oracle has introduced some parameters to play with this functionality over the yesrs. This is the short easy story. There are some fun complicated scenarios that arise during concurent access to a block by multiple users.

Best
Kyle

In article <9617r0$76n$1_at_nnrp1.deja.com>,   steveee_ca_at_my-deja.com wrote:
> Hi,
>
> I don't have a reference handy, but as I recall, when a commit is
 issued
> the transaction is marked with a commit record. The non-existence of
> this record is sufficient for Oracle to know it should be rolled back.
>
> Hth,
>
> Steve
>
> In article <9612sg$25r$1_at_nnrp1.deja.com>,
> letokai_at_my-deja.com wrote:
> > In :
> >
> > Oracle8i Administrator's Guide
> > Release 8.1.5
> > 6 Managing the Online Redo Log
> > chapter : Online Redo Log Contents
> >
> > I've read this :
> > "...Redo records can also be written to an online redo log file
> > before the corresponding transaction is committed. If the redo log
> > buffer fills, or another transaction commits, LGWR flushes all
> > of the redo log entries in the redo log buffer to an online
 redo log
> > file, even though some redo records may not be committed. If
> > necessary, Oracle can roll back these changes."
> >
> > If a rollback occurs how does Oracle do to process it?
> > I mean it must have stored somewhere the information that a
> > modification has not been commited yet.
> > Where is stored this information?
> >
> > Sent via Deja.com
> > http://www.deja.com/
> >
>
> Sent via Deja.com
> http://www.deja.com/
>

Sent via Deja.com
http://www.deja.com/ Received on Fri Feb 09 2001 - 13:38:56 CST