Rollback segments [Oracle]

In order for a rollback segment to be useful, it must be online. You can achieve this with

ALTER ROLLBACK SEGMENT users ONLINE;

Beware, however, if you shutdown and restart the database, the rollback segment will be offline unless you added the rollback's segment name to the database's parameter file:

rollback_segments=(rb_users, rb1, rb2, rb3, rb4)

• A transaction can only use one rollback segment to store its rollback record.
• The same rollback segment can be used by multiple transactions.
• No extent of a rollback segment is overwritten while a transaction that is not commited has written to that rollback segment.

The difference beetween private and public rollback segments is important only for the parallel server option: a public rbs can be brought online by any instance in a parallel server environment while a private one can only be aquired by the instance specifying the rbs in its init file through the rollback_segments.

CREATE ROLLBACK SEGMENT rb1 STORAGE(INITIAL 1M next 1M minextents 20)
  tablespace rbs;

If there are too few rollback segments it will cause a " undo header contention" If the rollback segments are too small it will cause a " undo block contention"

Rollback Data must be backed up.

Rollback segments belong to sys, irrespective of who creates them. They are not accessible to ordinary users.