Recovery Manager (RMAN) is a utility in Oracle that is generally used to backup, restore or recover database files. It performs backup and recovery by using database server sessions. The metadata about its operation may be stored in a control file of the target database or in recovery catalog schema in Oracle database. By default, RMAN automatically stores the metadata in the control file of the target database; so using a recovery catalog database is fully optional though it is encouraged to use a recovery catalog database so that if the production machine fails, we have the operations information in the catalog on a separate machine.

Before knowing how to use RMAN for backup and recovery, one should know why backup of files is required. One should remember that backup and recovery are one of the most important aspects in database administration. What is backup and recovery? It is a copy of the data which includes important parts of a database like redo log files, control files and data files. Normally back up is divided into two parts: physical backup and logical backup. Physical backup is the copy of physical database files. Logical backup contains data that is exported using SQL commands and stored in a binary file. Restoring a database generally refers to various operations in restoring, rolling forward and rolling back a backup. Regular backups prevent the loss of data and damage caused due to failure of disk drives, virus attack, power outrages and other network problems. No one is completely sure about when the failures may occur. One of the important reasons for failure is user error which may cause corruption or loss of valuable data. Another reason is media failure which can occur when the media, where the data files or transaction logs are stored, fails. Backup operations which are planned properly beforehand make the recovery process of the data very simple. Like this, there can be a number of reasons of failure.

At a minimum, the environment for RMAN requires:

·         The target database that is required to be backed up

·         The RMAN client

Other optional components are:

·         Media Management Software

·         Flash Recovery Area

·         Recovery Catalog Database

Target Database

It is the database which needs to be backed up, restored or recovered using RMAN. The target database's control file is used by RMAN to gather information about the target database and to store information about RMAN's own operations.

RMAN Client

It is here where RMAN commands and SQL statements can be issued for performing backup and restoring operations. RMAN client uses Oracle Net to connect to the target database. So locating a RMAN client to any host that is connected to the target host through Oracle Net will do the job.

The above two are the required components for performing backup or restoring using RMAN. The components described below are the optional components.

Media Management Software

Media management software is used by RMAN to handle sequential media devices (for example: tape libraries, etc.).The media manager controls these devices during backup and recovery operations. It manages the loading, labeling, unloading of media, among all other functions performed by it. Actually, what happens during backup or restore operation, RMAN client get connected to target instance and it is this instance which is directed to communicate with the media manger. So instead of having a direct communication between RMAN client and media manager, the communication happens through the target instance.

Flash Recovery Area

RMAN has a feature known as Automatic Disk-based Backup and Recovery that manages all the related files of backup and recovery in a flash recovery area. This feature thus simplifies managing disk space and files related to backup and recovery operations. The flash recovery area parameter can be set using:

·         db_recovery_file_dest – This parameter determines the location of the flash recovery area.

·         db_recovery_file_dest_size – This parameter determines how much space can be used by Oracle in the flash recovery area.

There can be a need to assign flash recovery area more space, which typically depends on:

·         The size of the database

·         The number of backups one wants or requires to keep

·         If the database is being run in ARCHIVELOG mode

Recovery Catalog Database

The Recovery catalog can be used to hold RMAN repository records, RMAN stored scripts, and the sequences of RMAN commands for common backup tasks. Actually, it can be more convenient to work with the centralized store of scripts than working with command files.

It stores the metadata about the target database and its backup and restores operations information. RMAN also stores information about its own configurational settings, the target database schema, etc. RMAN repository information can be viewed with the help of LIST, SHOW, and REPORT commands. RMAN repository data is stored in the control file of the target database. The initialization parameter, CONTROL_FILE_RECORD_KEEP_TIME, is responsible for controlling the time for how long the backup records are to be kept in the control file after which those records can be re-used to hold information about more recent backups. However, the repository can also be kept in a recovery catalog as mentioned earlier.

With RMAN one can perform various types of backup as shown below.

·         Open or closed – Open backup is the backup taken when the database is open: a backup of online, read/write data files. Closed backup is the backup of any part of the target database when it is mounted but not open.

·         Full or incremental – A backup of type full is the back up of a data file that contains every allocated block in the file being backed up. And an incremental backup is either a level 0 backup or a level 1 backup.

·         Consistent or inconsistent – A consistent backup is the backup taken when the database is mounted but is not open after a normal shutdown. Whereas an inconsistent backup is the backup taken of any part of target database when the database is open or when a crash occurred, etc.

Backup database using RMAN

In non-archive mode, using dos prompt type:

RMAN

the RMAN prompt gets displayed.

Listing 1

RMAN>Connect Target 
Connect to target database:Xxx<Dbid:123456789>

This is using the database of target file rather than the recovery catalog.

A Dbid stands for Database Identifier which is a unique identifier and found in all datafile headers and control files. It is used to identify a file belongs to which database.

A simple example of taking backup in non-archived mode:

Listing 2

shutdown immediate; //shuts down the database
startup mount;
backup database; //starts backing up the database
alter database open;

In archivelog mode, the same command can also be fired to backup the whole database.

Listing 3

backup database plus archivelog;

The backup command can backup the following type of files:

·         Database (which includes all datafiles as well as the current control file and current server parameter file)

·         Tablespaces, except for locally-managed temporary tablespaces

·         Current server parameter file

·         Current control file

·         Backup sets

·         Current datafiles

·         Archived redo logs

RMAN does not back up the following:

·         Online redo logs

·         Transported tablespaces before they have been made read/write

·         Client-side initialization parameter files, etc

Restore database using RMAN

Restoring and recovering a database using RMAN uses even more simple commands than backup.

Listing 4

restore database;
recover database;

RMAN has the intelligence to find out which datafiles are to be recovered or restored and the location of backed up files.

·         Automatic detection of block corruption and its repairing-Block Media Recovery allows fixing of a corrupted block, detected on backup, by RMAN while the datafile is online and the non-affected files continue to be available for selecting, updating, etc. which results in increased data availability and reduction in mean time to recover.

·         Comprehensive reporting- Users can easily retrieve information about all the currently executing and completed backup jobs, details on all backed up files and backup sets which are not being used anymore using special V$ views.

·         Intelligent monitoring of disk consumption- In Oracle database 10g Release 1, Flash Recovery Area allows administrators to setup notifications on disk space usage and automate obsolescence of expired backup sets, via RMAN client command-line or enterprise manager interfaces.

·         Optimization of performance and space-saving operations- RMAN provides high backup and restores data streaming performance with the help of its intimate knowledge of Oracle block structures. RMAN backs up only blocks that are in use or have ever been used, by default, when creating backup sets and saves disk space by merging blocks into as few backup pieces as necessary.

·         Fine-granular data operations- RMAN provides fine-granular, on and offline backups at the database, tablespace, datafile, control file, archive log, and block level.

Some of the disadvantages of using RMAN are:

·         If the location of the backup set is changed, it is required to register it using RMAN; otherwise it would result in hanging situations when trying to restore backup.

·         There is no relevant method to know whether during recovery, database restore is going to fail because of missing archive log file.

·         If tape backup is used, media management is compulsory.

·         Incremental backups though used to consume less space, are often slower as they use to read the whole database to find the changed blocks.

Backups were not so easy before Oracle 9i version. It was required to allocate a channel compulsorily for taking backups. The syntax was also complex. RMAN provides a simple and easy way of backing up the database without any complex syntax which is what makes it more popular.

By Debjani Mallick