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Unit 11: Recovery System

The time and efforts needed to recover is proportional to number of changes made. A process Notes
called check pointing is used to minimize the time needed to recover.

11.1.3 Overview of ARIES

Figure 11.1

ARIES

Analysis Phase Redo Phase Undo Phase

ARIES is, an algorithm for recovering from crash, that uses no-force, steal approach.

1. Analysis Phase: It analyses the buffer pool to identify the active transactions and dirty
pages.
2. Undo Phase: If the modified data is loaded into disk before a transaction commits, then it
must undo the modification in case of crash.
3. Redo Phase: It must restore the data which it was before the crash. This is done if the data
modified by committed transaction is not loaded onto the disk.
4. Rollback: All log records are stored in a linked list and to operate rollback, the linked list
is accessed in reverse order.
11.2 Failure Classification

There are various types of failure that may occur in a system, each of which needs to be dealt
with in a different manner. The simplest type of failure is one that does not result in the loss of
information in the system. The failures that are more difficult tom deal with are those that result
in loss of information. Various types of failure are:

Transaction Failure

There are two types of errors that may cause a transaction to fail:
1. Logical error: The transaction can no longer continue with its normal execution because of
some internal condition, such as bad input, data not found, overflow, or resource limit
exceeded.
2. System error: The system has entered an undesirable state (for example, deadlock), as a
result of which a transaction cannot continue with its normal execution. The transaction,
however, can be executed at a later time.

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. The
content of nonvolatile storage remains intact, and is not corrupted.

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