RE: Some rollback doubts

Rajesh:

I am not sure which document you are referring here. If that documentation says more than one transaction can not use a rollback segment data block means, I would say the documentation is incorrect in this case. But this can not happen concurrently. IT can happen serially.

Each rollback segment's header will have something called free extent pool, which will link at least 5 undo segment blocks which have more than 400 bytes
of free space. These blocks will be used to store undo information for the subsequent transactions. This 'free extent pool' will be clearly visible in the segment header dumps.

You can dump the rollback segment header by using the alter system dump undo header 'undo_segment_name' command. However this free extent pool just keep 5 undo blocks with free space. I hope this answers both of your questions..

Best Regards,
K Gopalakrishnan

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>From the docs : Many active transactions can write concurrently to a single
rollback segment--even the same extent of a rollback segment; however, each data block in a rollback segment's extent can contain information for only a single transaction.

1. Now, when oracle writes to the last extent in the cycle, and sees that the first extent has an active transaction, it allocates a new extent. Well, why should it? Cant it write to the first extent?
2. When a new transaction needs to use the rollback segment, and say it gets allocated to a rollback segment with a few other transactions, how does it decide which extent to use from the cycle of extents?

Thanks
Raj

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