Database Reference
In-Depth Information
Rolling Forward
The first step of recovery is to
roll forward
, which is to reapply to the datafiles all of the
changes recorded in the redo log. Rolling forward proceeds through as many redo log files
as necessary to bring the datafiles forward to the required time.
If all necessary redo information is online, Oracle rolls forward automatically when
the database starts. After roll forward, the datafiles contain all committed changes as well
as any uncommitted changes that were recorded in the redo log.
Rolling Back
The roll forward is only half of recovery. After the roll forward, any changes that were not
committed must be undone. After the redo log files have been applied, then the undo
records are used to identify and undo transactions that were never committed yet were
recorded in the redo log. This process is called
rolling back
. Oracle completes this step
automatically.
21.4.6 Oracle's Backup and Recovery Solutions
There are two methods for performing Oracle backup and recovery: Recovery Manager
(RMAN) and user-managed backup and recovery. RMAN is a utility automatically
installed with the database that can back up any Oracle8 or later database. RMAN uses
server sessions on the database to perform the work of backup and recovery. RMAN has
its own syntax and is accessible either through a command-line interface or though the
Oracle Enterprise Manager GUI. RMAN comes with an API that allows it to function with
a third-party media manager.
One of the principal advantages of RMAN is that it obtains and stores metadata
about its operations in the control file of the production database. You can also set up an
independent recovery catalog, which is a schema that contains metadata imported from
the control file, in a separate recovery catalog database. RMAN performs the necessary
record keeping of backups, archived logs, and so forth using the metadata, so restore and
recovery is greatly simplified.
An alternative method of performing recovery is to use operating system commands
for backups and SQL*Plus for recovery. This method, also called user-managed backup
and recovery, is fully supported by Oracle Corporation, although use of RMAN is highly
recommended because it is more robust and greatly simplifies administration.
Whether RMAN is used, or user-managed methods, physical backups can be
supplemented with logical backups of schema objects made using the Oracle Export
utility. The utility writes data from an Oracle database to binary operating system files.
You can later use Oracle Import to restore this data into a database.
