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Lesson 4 Fast-start on-demand rollback
Objective Describe the steps to enable Fast-start on-demand Rollback

Fast-Start On-Demand Rollback

Rollback segments record the transactions that must be undone during certain database operations. These segments are used to identify and undo transactions that were never committed, but were either saved to the datafiles before the failure, or were applied to the database during the roll forward. This process is called rolling back or transaction recovery and Oracle can roll back multiple transactions simultaneously. All the transactions that were active during or before the failure of a database are marked DEAD during recovery, and can have locks on certain records. Instead of waiting for the DEAD transactions to be rolled back, the new transactions can recover the blocking transactions to obtain the locks they need. Fast-start-on-demand rollback recovers the necessary locks, leaving the rest of the DEAD transaction to be recovered in the background.
When a dead transaction holds a row lock on a row that another transaction needs, Fast-Start On-Demand Rollback immediately recovers only the data block under consideration, leaving the rest of the dead transaction to be recovered in the background. This improves the availability of the database for users accessing data that is locked by large dead transactions. If Fast-Start Rollback is not enabled, the user would have to wait until the entire dead transaction was recovered before obtaining the row lock.

Fast-Start means never having to wait for a long roll back

The availability of large dead transactions is improved for users trying to assess the locked data. If Fast-Start rollback were not enabled, the user would have to wait until the entire dead transaction was recovered before obtaining the row lock. Consequently, new transactions do not have to wait until all parts of a long transaction are rolled back. This increases the efficiency of the recovery of an instance.
Oracle performs on-demand rollback automatically. You do not need to set any parameters or issue commands to use this feature.
The next lesson demonstrates how to implement parallel rollback with the fast-start parallel rollback feature.

Fast-Start Parallel Rollback

Fast-Start Parallel Rollback allows a set of transactions to be recovered in parallel using a group of server processes. This technique is used when SMON determines that the amount of work it takes to perform recovery in parallel is less than the time it takes to recovery serially.