Improved Backup Resource Management. Oracle 9i added
new features to help DBAs to automatically manage backup file retention
with the RETENTION POLICY directives
of the CONFIGURE
command set. Oracle 10g has improved RMAN resource management even further with
the DURATION
directive of the BACKUP
command: It is now possible to tell RMAN exactly how much system resources
should be allocated to accomplish a backup task so that it completes within a
specified time frame.
For example, my client's primary production database is
scheduled to begin at 00:15 every day, and needs to complete before batch
processing commences at 03:00 every day. In my daily backup RMAN script, I can
specify that the backups must complete in 2.5 hours, and RMAN will begin
backing up the specified database files:
BACKUP DURATION 2:30 DATABASE;
If the backup cannot complete within this time frame, the
RMAN script being executed will return an error and terminate the backup - not
necessarily a desirable outcome! However, if I specify the PARTIAL
directive, RMAN will not return an error, but will back up as many files as it
can in that time frame, starting with the least recently used backed-up files
(a feature of using DURATION):
BACKUP DURATION PARTIAL 2:30 DATABASE FILESPERSET 1;
In this case, any files that could not be backed up will be
logged as errors from the RMAN script, but all other backup files will be
retained. Oracle does recommend setting FILESPERSET to 1 when using DURATION
PARTIAL to insure that any files for which backups succeeded are retained. I
can also tune backup performance so RMAN will try to complete the backups as
quickly as possible by specifying the MINIMIZE TIME directive:
BACKUP DURATION PARTIAL 2:00 MINIMIZE TIME DATABASE FILESPERSET 1;
If I specify the MINIMIZE LOAD directive, on the other hand,
RMAN will instead "stretch out" backup operations so that fewer resources are
utilized during that time:
BACKUP DURATION PARTIAL 3:30 MINIMIZE LOAD DATABASE FILESPERSET 1;
Server Parameter File (SPFILE) AutoBackups. Oracle 9i
added the ability to configure automatic control file backups to occur whenever
specific RMAN operations happened, or when the DBA performed a significant
modification of the database's logical or physical structure that affected the
control file (e.g. adding a new tablespace, or renaming a datafile).
Oracle 10g expands this feature to include the auto-backup
of the database's server parameter file - the binary copy of the
initialization parameter file -- as well. Though I have to admit that I am
still a fan of the initialization parameter file - old habits do die hard, dang
it! - it is obvious that Oracle views the SPFILE as the future basis for
controlling database parameter configuration.
Enhanced BEGIN BACKUP. Finally, here is a neat
enhancement for user-managed backups: The BEGIN BACKUP command that is used to
take tablespaces offline one at a time has been enhanced so that all of the
database's tablespaces can be taken offline at once:
-- Take all datafiles offline before starting user-managed backup
ALTER DATABASE BEGIN BACKUP;
-- Bring all datafiles back online after completing user-managed backup
ALTER DATABASE END BACKUP;
Though our shop uses RMAN for all production database
backups, this command certainly has value for smaller but no less
mission-critical databases like OEM or RMAN recovery catalog repositories.
Automatic Channel Failover. For those of you who
create RMAN backups directly on tape via a Media Management Layer (MML), Oracle
10g adds a new feature: If multiple channels have been allocated for the backup
step, but any one channel fails during that step, RMAN will automatically try
to use one of the other available channels to continue processing the backups.
Though I have had limited experience with using MML in conjunction with RMAN,
this feature appears to increase the flexibility and stability of directly
backing up to alternate media.
Restoration and Recovery Enhancements
RESTORE Failover. Oracle 10g has also significantly
improved the restoration process during initial restoration and recovery
efforts:
-
If RMAN should detect a damaged or missing backup file, it will
automatically attempt to locate another usable copy of the image copy or
backup piece, either at the default location or at an alternate
multiplexed location.
-
If it cannot find a usable current copy, it then looks at prior
backup pieces or image copies and attempts to restore from those files.
-
If RMAN cannot locate any appropriate backup or image copy, only
then will it issue an error and terminate the RMAN session.
RESTORE ... PREVIEW. If you have ever wondered exactly
what backup files or image copies RMAN will use to perform restoration, Oracle
10g now offers the RESTORE ... PREVIEW command set to show exactly what backup
pieces or image copies RMAN plans to utilize.
For example, if I wanted to explore exactly what RMAN will
choose if I want to restore the database's SYSTEM tablespace, from within an
RMAN session, I can issue the RESTORE DATAFILE 1 PREVIEW; command and RMAN will
return the following output:
See Listing 1.4 for additional examples of this
command set.
Automatic Creation of Missing Datafiles. Consider
this scenario: Your junior DBA has just added a new tablespace to the
production database, but she neglected to take a full backup of the database
immediately after adding the tablespace. Then, as luck would have it, a media
failure occurs on the same disk where the new tablespace's datafile resides.
Here's the good news: With Oracle 9i, it's definitely
possible to recreate the datafile for the new tablespace - as long as all the
archived redo logs and online redo logs that were generated since the creation
of the new tablespace are available, that is. Once the datafile has been taken
offline, the ALTER DATABASE CREATE DATAFILE <datafile
name>; command is issued to recreate the datafile. Then
the RECOVER DATAFILE <datafile
name>; command is issued to recover the datafile, and the datafile's
tablespace can be brought back online.
Moreover, here is the better news: Oracle 10g is now
smart enough to handle this situation without DBA intervention. If the database
encounters a redo log entry for the creation of the datafile, it will
automatically add the new datafile to the database.
Other Enhancements
Improved Access To RMAN Metadata. Oracle 10g provides
some new dynamic views that offer a DBA the ability to see what's really
happening during and after a set of RMAN tasks have been completed, thus saving
the effort of having to constantly monitor a command window or log file to
determine their status.
V$RMAN_OUTPUT
will show the status of an ongoing RMAN job. For example, here is some
sample output from the example of the full database image copy backup run:
Results of currently active Recovery Manager sessions
Activity
----------------------------------------------------------------
connected to target database: ZDCDB (DBID=1863541959)
using target database controlfile instead of recovery catalog
allocated channel: dbkp1
channel dbkp1: sid=145 devtype=DISK
Starting backup at 20-NOV-04
channel dbkp1: starting datafile copy
input datafile fno=00001 name=C:\ORACLE\ORADATA\ZDCDB\SYSTEM01.DBF
output filename=C:\ORACLE\RMANBKUP\DATA_D-ZDCDB_I-1863541959_TS-
SYSTEM_FNO-1_2OG5IGDQ tag=TAG20041120T114042 recid=2 stamp=54272
channel dbkp1: datafile copy complete, elapsed time: 00:01:19
channel dbkp1: starting datafile copy
input datafile fno=00013 name=C:\ORACLE\ORADATA\ZDCDB\SYSAUX01.D
BF
... (Some detail removed for brevity) ...
channel dbkp1: datafile copy complete, elapsed time: 00:00:03
Finished backup at 20-NOV-04
released channel: dbkp1
66 rows selected.
In addition, V$RMAN_STATUS lists the historical status
of all RMAN jobs. Here is the resulting output from my (not always successful!)
experiments with image backups:
Results of most recent Recovery Manager sessions
Command/
TimeStamp Session Action Status
-------------------- -------- -------- ------------------------
2004-11-20T11:40:33 COMMAND BACKUP COMPLETED
2004-11-20T11:40:33 SESSION RMAN COMPLETED
2004-11-20T11:39:50 SESSION RMAN COMPLETED WITH ERRORS
2004-11-20T11:39:50 COMMAND BACKUP FAILED
2004-11-20T11:33:49 COMMAND BACKUP FAILED
2004-11-20T11:33:49 SESSION RMAN COMPLETED WITH ERRORS
6 rows selected.
See Listing 1.5 for the queries used to create this
output.
Improved Recovery Catalog Maintenance. Oracle 10g
offers a new catalog maintenance command, UNREGISTER DATABASE, to remove all information
about an Oracle database from an RMAN repository.
Dropping a Database Completely. If you really must
drop an entire database, the new DROP DATABASE command will remove all of the
specified database's physical files, including control files, datafiles,
online redo log members, and server parameter files (if any exist). Note that
the database must be mounted in exclusive, restricted mode for this command to
succeed.
Conclusion
Oracle 10g's new Recovery Manager features greatly expand
the flexibility and reliability of any Oracle DBA's tool kit for disaster
recovery planning, backup strategies and failure recovery scenarios. And I've
just scratched the surface! As promised, the next article in this series will
focus on one of the most intriguing new availability features: Flash
Backup and Recovery.
References and Additional Reading
While there is no substitute for direct experience, reading
the manual is not a bad idea, either. I have drawn upon the following Oracle
10g documentation for the deeper technical details of this article:
B10734-01 Oracle Database
Backup and Recovery Advanced User's Guide
B10735-01 Oracle Database Backup
and Recovery Basics
B10750-01 Oracle Database
New Features Guide
B10770-01 Oracle Database
Recovery Manager Reference
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