Synopsis. Oracle 10g offers significant
enhancements that help insure the high availability of any Oracle database,
especially in the arena of disaster recovery. This article – the first in a
series – concentrates on several new features available for backup,
restoration, and recovery of Oracle databases, especially when using Oracle
Recovery Manager (RMAN).
If you have read my earlier
article about setting up disaster recovery for Oracle databases, you
already know that I sincerely enjoy experimenting with the myriad robust
features of Oracle Recovery Manager (RMAN). I am willing to bet that any
seasoned Oracle DBA sighs knowingly and thankfully when she thinks of a
potentially disastrous loss of data that has been averted by a well-planned
backup and recovery strategy that incorporates RMAN.
Oracle 10g expands significantly the RMAN backup,
restoration, and recovery features that I have grown to appreciate. Flash
Backup and Recovery appears to be the most exciting improvement, and I will
cover that in greater detail in the next article in this series, but for now,
this article will focus on numerous significant features that beg for
illustration.
Backup Enhancements
Expanded Image Copying Features. A standard RMAN backup
set contains one or more backup pieces, and each of these pieces
consists of the data blocks for a particular datafile stored in a special
compressed format. When a datafile needs to be restored, therefore, the entire
datafile essentially needs to be recreated from the blocks present in the
backup piece.
An image copy of a datafile, on the other hand, is
much faster to restore because the physical structure of the datafile already
exists. Oracle 10g now permits image copies to be created at the database, tablespace,
or datafile level through the new RMAN directive BACKUP AS COPY. For example, here is a command
script to create image copies for all datafiles in the entire database:
RUN {
# Set the default channel configuration. Note the use of the
# %U directive to insure unique file names for the image copies
ALLOCATE CHANNEL dbkp1 DEVICE TYPE DISK FORMAT ‘c:oraclermanbkupU%’;# Create an image copy of all datafiles in the database
BACKUP AS COPY DATABASE;
}
See Listing 1.1 for additional RMAN script examples
of this new feature.
Incrementally Updated Backups. As I explained in the
previous section, it is now much simpler to create image copy backups of the
database. Another new Oracle 10g feature, incrementally updated backups,
allows me to apply incremental database changes to the corresponding image copy
backup – also known as rolling forward the datafile image copy — of any
datafile in the database. Since image copy backups are much faster to restore
in a media recovery situation, this new feature gives me the option to have
updated image copies ready for restoration without having to recreate the
image copies on a regular basis.
To utilize this feature, I will need to use the new BACKUP … FOR RECOVER OF COPY
command to create the incremental level 1 backups to roll forward the changes
to the image copy of the datafiles, and use the new RMAN RECOVER COPY OF DATABASE command
to apply the incremental backup to the image copies of the datafiles. Note that
the TAG directive becomes extremely important to this implementation, as it is
used to identify to which image copies the changes are to be rolled forward.
Here is a script that illustrates a daily cycle of creation
and application of the incrementally updated backups. This would be appropriate
for a database that has sufficient disk space for storage of image copies, and
has a relatively high need for quick restoration of media:
RUN {
# Roll forward any available changes to image copy files
# from the previous set of incremental Level 1 backups
RECOVER
COPY OF DATABASE
WITH TAG ‘img_cpy_upd’;# Create incremental level 1 backup of all datafiles in the database
# for roll-forward application against image copies
BACKUP
INCREMENTAL LEVEL 1
FOR RECOVER OF COPY WITH TAG ‘img_cpy_upd’
DATABASE;
}
Though this appears a bit counter-intuitive at first, here
is an explanation of what happens during the initial run of this script:
-
The RECOVER command actually has no effect, because it cannot
find any incremental backups with a tag of img_cpy_upd. -
However, the BACKUP command will create a new Incremental
Level 0 backup that is labeled with a tag of img_cpy_upd because no backups have been created
yet with this tag.
And during the second run of this script:
-
The RECOVER command still will have no effect, because it cannot
find any Level 1 incremental backups with a tag of img_cpy_upd. -
The BACKUP command will create its first Incremental Level
1 backup that is labeled with a tag of img_cpy_upd.
But during the third and subsequent runs of
this script:
-
The RECOVER command finds the incremental level 1 image copy
backups from the previous night’s run tagged as img_cpy_upd, and applies them to the existing datafile image
copies. -
The BACKUP command will create the next Incremental Level
1 backup that is labeled with a tag of img_cpy_upd.
After the third run of this script, RMAN would then choose
the following files during a media recovery scenario: the image copy of
the database for tag img_cpy_upd from the
previous night, the most recent incremental level 1 backup, and all archived
redo logs since the image copy was taken. This strategy offers a
potentially quick and flexible recovery, since the datafile image copies will
be relatively quick to restore, and the incremental level 1 backup plus all
archived redo logs can be used to perform either a point-in-time or a
complete recovery.
See Listing 1.2 for an example of how this new
feature could be implemented for a weekly backup strategy.
Improved Incremental Backup Performance With Change
Tracking. Another new Oracle 10g optional feature, change tracking,
promises to improve the performance of incremental backup creation
significantly. When an incremental backup was being taken prior to 10g, all the
blocks in each datafile being backed up needed to be scanned to determine if the
block had changed since the last incremental backup to determine if it needed
to be included in the new incremental backup.
With the new change tracking feature enabled, however, now
only the first Level 0 incremental backup needs to be completely scanned, and
the IDs of any changed blocks are written instead to a change tracking file.
All subsequent incremental backups will query the change tracking file to
determine if there are any changed blocks that need to be backed up. Oracle
automatically stores enough incremental backup metadata to insure that any of
the eight most recent incremental backups can be used as the “parent” of a new
incremental backup.
Each Oracle database has only one change tracking file, and
if the database has been configured for Oracle Managed Files (OMF) it will be
automatically created based on the specification for DB_CREATE_FILE_DEST. However, if OMF is not
enabled for the database, the location of the change tracking file can be
specified manually. The initial size of the change tracking file is 10MB, and
it grows in 10MB increments, but Oracle notes that the 10MB initial extent
should be sufficient to store change tracking information for any database up
to one terabyte in size.
If the location needs to be moved, change tracking can be
disabled, and a new change tracking file can be created, but this causes the
database to lose all change tracking information. Moreover, unfortunately the change
tracking file cannot be moved without shutting down the database, moving it
with the appropriate ALTER DATABASE RENAME FILE <filename> command, and
then restarting the database.
Oracle does recommend that this feature be activated for any
database whose disaster recovery plan utilizes incremental backups of differing
levels. Oracle also notes that theirs is a small performance hit during normal
operations, but that hit should be discounted against the need to avoid scans
of datafiles during restoration and recovery operations.
See Listing 1.3 for more extensive RMAN script examples
of this new feature.
