Introduction to database management systems

Database failures require recovery which can be divided into three categories:

• Instance failures are result of an internal exception within DBMS, an operating system failure, or other software-related database failure. In some cases, an instance failure can result in corruption of data that requires a recovery, but usually such failures do not damage data, so DBMS simply needs to be restarted to reestablish normal operations.
• Application (or transaction) failures occur when programs or scripts are run at wrong time, using wrong input, or in wrong order. An application failure usually results in corrupt data that requires a database restore or recovery. The sooner an application failure is identified and corrected, smaller amount of damage to database will be.
• Media failure is likely to damage data, too. Media failure includes damage to disk storage devices, file system failures, tape degradation or damage, and deleted data files. Although less common in practice, damaged memory chips also can cause data corruption. After a media failure, database will likely be in a state where valid data is unreadable, invalid data is readable, or referential integrity is violated. 

A fundamental component of a database backup and recovery plan is creating backup copies of data. When an error occurs that damages integrity of database, a backup copy of data can be used as the basis to recover or restore database. Backing up databases involves making consistent copies of your data, usually in form of image copies, which are output of a COPY utility.

Full vs. Incremental Backups 

Two types of image copy backups can be taken: full and incremental. 

A full image copy backup is a complete copy of all data in database object at the time image copy was run. 

An incremental image copy backup, sometimes referred to as a differential backup, contains only the data that has changed since the last full or incremental image copy was made. The advantage of taking an incremental backup rather than a full backup is that it can sometimes be made more quickly, and it occupies less space on disk or tape. The disadvantage is that recovery based on incremental copies can take longer because, in some cases, the same row is updated several times before the last changes are restored.