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When a transaction starts changing some data in a block, it first writes
the old image of the data to a rollback segment. The information stored in a
rollback segment is used for two main purposes: to provide the information necessary
to roll back a transaction and to support multiversion read consistency.
A rollback segment is not the same as a redo log. The redo log is used to log all
transactions to the database and to recover the database in the event of a system
failure, while the rollback segment provides rollback for transactions and read
consistency.
Blocks of rollback segments are cached in the SGA just like blocks of tables and
indexes. If rollback segment blocks are unused for a period of time, they may be
aged out of the cache and written to the disk.
Chapter 8 discusses Oracle??™s method for concurrency management,
multiversion read consistency. This method uses rollback segments to
retrieve earlier versions of changed rows. If the required blocks are no
longer available, Oracle delivers a ???snapshot too old??? error.


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