What is Aries recovery algorithm.?

Answer 1

three main principles lie behind ARIES:

- Write ahead logging: Any change to an object is first recorded in the log, and the log must be written to stable storage before changes to the object are written to disk.

- Repeating history during Redo: On restart after a crash, ARIES retraces the actions of a database before the crash and brings the system back to the exact state that it was in before the crash. Then it undo the transactions still active at crash time.

- Logging changes during Undo: Changes made to the database while undoing transactions are logged to ensure such an action isn't repeated in the event of repeated restarts.

The ARIES recovery procedure consists of three main steps :

- Analysis : It identifies the dirty (updated) pages in the buffer and the set of transactions active at the time of crash. The appropriate point in the log where REDO operation should start is also determined.

- REDO phase : It actually reapplies updates from the log to the database. Generally the REDO operation is applied to only committed transactions. However, in ARIES, this is not the case. Certain information in the ARIES log will provide the start point for REDO, from which REDO operations are applied until the end of the log is reached. Thus only the necessary REDO operations are applied during recovery.

- UNDO phase : The log is scanned backwards and the operations of transactions that were active at the time of the crash are undone in reverse order. The information needed for ARIES to accomplish its recovery procedure includes the log, the transaction table, and the dirty page table. In addition, checkpointing is used.