Ch 06: Redo and Undo

Database restore and recovery is the one thing that a DBA is not allowed to get wrong.

ARCn copies online redo log (ORL) to archived redo log files.

Without redo logs the database would not offer any more protection than a filesystem.

Each node in a RAC has its own ORL and UNDO tablespace.

The database is logically restored to the way it was – any changes are logically undone – but the data structures, the database blocks themselves, may well be different after a rollback.

Deferred segment creation (default 11.2+)

Undo segments are protected by REDO.

Before DBWn can write any of the blocks that are changed to disk, LGWR must flush (to disks) the REDO information related to those blocks.

This includes to the UNDO needed in case of database failure during an incomplete transaction.

Crash recovery - roll forward all REDO then rollback all incomplete transactions.

ORL not read except for archival during normal processing.

A COMMIT is generally a very fast operation, regardless of the transaction size.

COMMIT:

PL/SQL commit opt suspended for distributed transaction, and DG in maximum available mode.

Would this be implied by maximum protection mode?

Rollbacks are expensive.