Physical Database Structures Used in Recovering Data

Before you begin to think seriously about a backup and recovery strategy, the physical data structures relevant for backup and recovery operations must be identified. These components include the files and other structures that constitute data for an Oracle data store and safeguard the data store against possible failures. Three basic components are required for the recovery of Oracle Database:

Data files

Oracle Database consists of one or more logical storage units called tablespaces. Each tablespace in Oracle Database consists of one or more files called data files, which are physical files located on or attached to the host operating system in which Oracle Database is running.

The data in a database is collectively stored in the data files that constitute each tablespace of the database. The simplest Oracle Database would have one tablespace, stored in one data file. Copies of the data files of a database are a critical part of any backup strategy. The sheer size of the data files is the main challenge from a VLDB backup and recovery perspective.

Redo Logs

Redo logs record all changes made to a database's data files. With a complete set of redo logs and an older copy of a data file, Oracle can reapply the changes recorded in the redo logs to re-create the database at any point between the backup time and the end of the last redo log. Each time data is changed in Oracle Database, that change is recorded in the online redo log first, before it is applied to the data files.

Oracle Database requires at least two online redo log groups. In each group, there is at least one online redo log member, an individual redo log file where the changes are recorded. At intervals, Oracle Database rotates through the online redo log groups, storing changes in the current online redo log while the groups not in use can be copied to an archive location, where they are called archived redo logs (or, collectively, the archived redo log). For high availability reasons, production systems should always use multiple online redo members per group, preferably on different storage systems. Preserving the archived redo log is a major part of your backup strategy, as it contains a record of all updates to data files. Backup strategies often involve copying the archived redo logs to disk or tape for longer-term storage.

Control Files

The control file contains a crucial record of the physical structures of the database and their status. Several types of information stored in the control file are related to backup and recovery:

  • Database information required to recover from failures or to perform media recovery

  • Database structure information, such as data file details

  • Redo log details

  • Archived log records

  • A record of past RMAN backups

Oracle Database data file recovery process is in part guided by status information in the control file, such as the database checkpoints, current online redo log file, and the data file header checkpoints. Loss of the control file makes recovery from a data loss much more difficult. The control file should be backed up regularly, to preserve the latest database structural changes, and to simplify recovery.