||Oracle Database Tips by Donald Burleson
A hot backup, or one taken while the
database is active, can only give a read-consistent copy; it doesn't
handle active transactions. You must ensure that all redo logs
archived during the backup process are also backed up. The hot
backup differs from the cold backup in that only sections of the
database are backed up at one time. This is accomplished by using
the ALTER command to modify a tablespace's status to backup. Be sure
that you restore the status to normal once the database is backed up
or else redo log mismatch and improper archiving/rollbacks can
While it is quite simple (generally
speaking) to do a cold backup by hand, a hot backup can be quite
complex, hence should be automated. The automated procedure should
then be thoroughly tested on a dummy database for both proper
operation and the ability to restore prior to its use on the
The following are limitations on hot, or
The database must be operating in
ARCHIVELOG mode for hot backups to work.
Hot backups should only be done during
off-hours or low-use periods.
During the hot backups, the entire block
containing a changed record, not just the changed record, is
written to the archive log, requiring more archive space for
The hot backup consists of three
The tablespace datafiles are backed up.
The archived redo logs are backed up.
The control file is backed up.
The first two parts have to be repeated for
each tablespace in the database. For small databases, this is
relatively easy. For large, complex databases with files spread
across several drives, this can become a nightmare if not properly
automated in operating system-specific command scripts. An example
of this type of a backup shell script is shown in Source 15.1.
As you can see, this is a bit more complex
than a full cold backup and requires more monitoring than a cold
backup. Recovery from this type of backup consists of restoring all
tablespaces and logs and then recovering. You only use the backup of
the control file if the current control file was also lost in the
disaster; otherwise, be sure to use the most current copy of the
control file for recovery operations.
In a number of computer facilities, backups
are kept close at hand, sometimes in the same room as the computer.
What would happen if a site disaster destroyed the computer room?
Not only the hardware, but all of the system backups and your data,
could be lost. The point is: Store backups in another building or
even totally off-site. This assures that come fire, flood, or
typhoon, you should be able to get backup one way or another.
See Code Depot