Undo is used in the rollback of transactions, and for database recovery, read consistency,
and flashback features. (Redo records, though, reside in the physical redo
logs; they store changes to data segments and undo segment data blocks, and they
hold a transaction table of the undo segments.) The undo retention period is now
self-tuned by Oracle based on how the undo tablespace is configured.
For your database release, check the documentation regarding optional initialization
parameters as these change from release to release. Some of them are described in the
following sections.
Deploying Physical Components
This section is not a substitute for Oracle??™s installation procedures, but it should provide
you with some practical guidance as you plan deployment of an Oracle database.
Deploying Physical Components | 39
Control Files
A database should have at least two control files on different physical disks. Without
a current copy of the control file, you run the risk of losing track of portions of your
database. Losing control files is not necessarily fatal??”there are ways to rebuild them.
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