Page 200 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Unit 11: Recovery System

Notes
Two-safe: This commit protocol proceeds if both the primary and backup site are active.
However, if only the primary is active, the transaction commits as soon as its commit log
record is written at the primary site. This protocol provides better availability than two-
very-safe, as it avoids problems of lost transactions in one-safe.

11.9 Summary

 The recovery mechanism is needed in database system to take care of failures.
 One Formatted: Centered can use either the log based or page based recovery schemes.
 Buffer management is an important issue for DBMS as it affects the process of recovery.

 Remote backup systems allow a copy of complete data.
11.10 Keywords

Deferred database modification: The deferred database modification scheme records all the
modifications to the log, but defers all the writes to after partial commit.

Disk failure: A disk block loses its content as a result of either a head crash or failure during a
data transfer operation.
Immediate database modification: The immediate database modification scheme allows database
updates on the stored database even of an uncommitted transaction.
Log records: Log records are buffered in the main memory, instead of being output directly to a
stable storage media.
Recovery: Recovery algorithms are techniques to ensure database consistency and transaction
atomicity and durability despite failures.

Storage media: Storage media can be distinguished by their relative speed, capacity, and resilience
to failure, and classified as volatile storage or nonvolatile storage.
System crash: There is a hardware malfunction, or a bug in the database soft ware or the operating
system, that causes the loss of the content of volatile storage, and brings transaction processing
to a halt.

11.11 Self Assessment

Fill in the blanks:
1. A transaction may fail because of hardware or a ................................. failure.
2. The modifications made to the data is called ...........................
3. ............................. systems guarantee that the failure of a single disk will not result in loss of
data.
4. ............................failure occurred in the midst of transfer, and the destination block has
incorrect information.
5. RAM stands for .............................
6. A database buffer can be implemented either, in an area of real main-memory reserved
for the database, or in the ..............................
7. Non-volatile storage such as ..............................

LOVELY PROFESSIONAL UNIVERSITY 193