Recovery Manager (RMAN) is an Oracle utility that
can back up, restore, and recover database files.
The product is a feature of the Oracle database
server and does not require separate installation.Recovery
Manager uses database server sessions to perform
the work of backup and recovery. It stores metadata
about its operations in the control file of the
target database and, optionally, in a recovery catalog
schema in an Oracle database.
RMAN was initially introduced in Oracle 8 and lot
of new features that made RMAN more powerful were
added in Oracle 8i. With the introduction of Oracle
9i, Oracle improved this tool to have many more
new features that make the database administrators
life much easier.
Persistent RMAN Configurations
A CONFIGURE command has been introduced in Oracle
9i, that lets you configure various features including
automatic channels, parellelism, backup options
etc., that can persist across sessions and be available
to any session. For example, the CONFIGURE command
may be used to store channel configurations and
RMAN will automatically assignt the channels as
needed without having to specify the channel allocation
as a part of the script. These automatic allocations
and options can be overridden by commands in an
RMAN command file.
Control File Auto Backups
Many of the database administrators stayed away
from using RMAN citing the reliance of RMAN on control
files / recovery catalog for backup and recovery
options. Now you need not worry about a lost recovery
catalog or a lost control file. Control File Auto
Backups give the dba a way to restore the backup
repository contained in the control file when a
control file and the recovery catalog are both lost.
For some of us using the nocatalog option this comes
in very handy when we have to recover without having
the control file that was used for the RMAN backup.
To use this feature, CONFIGURE CONTROLFILE AUTOBACKUP
should be set to ON, and RMAN will automatically
perform a control file autobackup after every BACKUP
or COPY command is issued whether its in a RUN Block
or at the command prompt. This control file backup
occurs in addition to any control file backup that
has been created as a part of the database backup.
Block Media Recovery
RMAN introduces a new blockrecover command to recovery
individual datablocks within a data file. This reduces
the Mean Time to Recovery ( MTTR ) for a large datafile
when individual blocks within the file are reported
as corrupt. Without this feature, even if a single
block is corrupted the dba has to restore the whole
datafile and recovery using the archived redo logs
that were generated since that backup was created.
This procedure is useful when a trace file or standard
output reveals that a small number of blocks within
a datafile are corrupt. A major restriction is that
you can only recover from a full RMAN backup and
incremental backups cannot be used for this type
of recovery. Also only complete recovery can be
performed on individual blocks cand you cannot stop
the redo application while the recovery is in progress.
Block Media recovery can be performed by using
a command at the RMAN command prompt similar to
the one given below.
BLOCKRECOVER DATAFILE 6 BLOCK 23 DATAFILE 12 BLOCK 9;
Oracle has enhanced RMAN in oracle 9i with lot
many features that make the job of the dba more
error free by enhancing the CHANGE, CROSSCHECK,
DELETE and LIST commands. Also RMAN automatically
switches out the online redo log before backing
up the archived redo logs to guarantee that the
backups can be restored to a consistent state. One
cool feature I like is the "NOT BACKED UP SINCE
" clause that allows RMAN to backup only those
files that were not backed up after a specified
time. So, f a backup fails partway through, you
can restart it and back up only those files that
were not previously backed up.