Saturday, October 10, 2009

Configuring Undo Space

Configuring Undo Space
Oracle needs undo space to keep information for read consistency, for recovery, and
for actual rollback statements. This information is kept in one or more undo

Oracle provides automatic undo management, which completely automates the
management of undo data. A database running in automatic undo management mode
transparently creates and manages undo segments. Oracle Corporation strongly
recommends using automatic undo management, because it significantly simplifies
database management and removes the need for any manual tuning of undo (rollback)
segments. Manual undo management using rollback segments is supported for
backward compatibility reasons.

Adding the UNDO TABLESPACE clause in the CREATE DATABASE statement sets up the
undo tablespace. Set the UNDO_MANAGEMENT initialization parameter to AUTO to
operate your database in automatic undo management mode.

The V$UNDOSTAT view contains statistics for monitoring and tuning undo space.
Using this view, you can better estimate the amount of undo space required for the
current workload. Oracle also uses this information to help tune undo usage in the
system. The V$ROLLSTAT view contains information about the behavior of the undo
segments in the undo tablespace.

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