BISQL – Laymen to SQL Developer # 38 – Recovery Techniques #1 – Introduction, Techniques for Recovering

Hi Folks,

This post is part of Series Database Management Systems

Currently running topic for this series is listed as below :

Series of Database Management Systems

>>Chapter 1 : DBMS [Database Management Systems]

>>Chapter 2 : Database Core Concepts and  Applications

>>Chapter 3 : Record Storage and Primary File Organization

>>Chapter 4 : Index Structures of Files

>>Chapter 5 : Entity-Relationship Model

>>Chapter 6 : Relational Algebra

>>Chapter 7 : SQL

>>Chapter 8 : Functional Dependencies and  Normalization For Relational Databases

>>Chapter 9 : Transaction Processing

>>Chapter 10 : Concurrency Control Techniques

>>Chapter 11 : Recovery Techniques<You are Here>

Continuing from my previous post on this series.

We are going to Cover the Following Points in this article

• Introduction
• Techniques for Recovering

Introduction

A computer system, like any other mechanical or electrical device is subjected to failure. The different reasons for such failure are disk crash, power failure, s/w error etc. In each of these cases, information may be lost. Therefore the database system is responsible for the restoration of the database to a consistent state just before the time of failure. To begin the original state of the database, the DBMS must keep information about the changes made by the various transactions in the system log.

Block disk cache: cached values on disk are stored by Block disk cache.. Like the Indexed Disk Cache, it keeps the keys in memory. The Block Disk Cache stores the values in a group of fixed size blocks.

System log: This contains events logged by any operating systems. For example, If any driver fails booting during the initial process, that event is record in the system log. Operating system finds it before hand the events that are logged by system components.

Transaction commit: Transaction commit is responsible for making all the data modifications permanent in the database. When transaction commit occurs the following are the observations:

· A commit record to indicate the modifications as permanent is written to the log.

· Depending upon the type of commit the log information in memory is simultaneously written to disk.

· Locks are released. This means the modifications can be viewable.

Techniques for Recovering

Recovery is a process whereby an image backup of the data base (has taken place, let’s say at 7.00 A.M.), and system failure has occurred at

2.00 P.M. This event causes inconsistency in the database. Say a system

had been used by 12 people between 7.00 A.M. and 2.00 P.M., everything these people did to the database was written to the redo logs. The recovery manager should redo the changes and get back the original status of the database.

By using achieved redo logs (their log files contain all the changes made to the database, we can completely recover the database without losing any data) we can get back the data up to the time of failure. Then using rollback segments it undoes any uncommitted transactions that were recorded in the redo logs. Thus the redo logs are a mirror image of the database.

The DBA should be responsible for bringing the database back to operation as quickly as possible and with little or no data loss. Conceptually, we can distinguish two main techniques for recovery from non-catastrophic failures (1) Deferred Update & (2) Immediate Update Recovery techniques.

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