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Crazy mistake in "Oracle Unleashed"

From: Jeremiah Wilton <jeremiah_at_wolfenet.com>
Date: Wed, 23 Sep 1998 21:56:36 -0700
Message-ID: <Pine.OSF.3.95.980923214703.10554A-100000@gonzo.wolfenet.com>

I found the following section in the "Oracle Uleashed" book. Please correct me if I am wrong that it is fundamentally wrong about how hot backups work. I must admit I started looking for this stuff after I heard some pretty wild theories about how hot backups work. The fun stuff starts in the third paragraph. My favorite part is the comment about the USS Nimitz. I'd say it is more like calling the USS Nimitz a potato.



From Oracle Unleashed 2nd Ed. (SAMS Pub.) ISBN: 0672311488

Hot Backups

Whereas a cold backup takes a backup of a database in a shutdown state, a hot backup enables you to take a backup of a database that has not been shut down. This is the most tedious backup method, but it is also the most flexible. It enables you to take backups of an active database. It ensures resource availability to end users and enables the DBA and the operations staff to recover the database.

Cold backups concentrate on copying all the physical files associated with a database instance. Hot backups, on the other hand,concentrate on the tablespace level. To do a hot backup, you must place every individual tablespace into a backup mode (by using the alter tablespace command), copy the physical database files that make up the tablespace, and take the tablespace out of backup mode (by using thealter tablespace command). You can issue these commands from Oracle Server*Manager or SQL*Plus. For example,

alter tablespace system begin backup;

alter tablespace system end backup;

When you place a tablespace in backup mode, the Oracle instance notes that a backup is being performed and internally compensates for it. As you know, it is impossible to make an authentic copy of a database file that is being written to. On receipt of the command to begin the backup, however, Oracle ceases to make direct changes to the database file. It uses a complex combination of rollback segments, buffers,redo logs, and archive logs to store the data until the end backup command is received and the database files are brought back in sync.

Simplifying a hot backup in this way is tantamount to classifying the USS Nimitz as a boat. The complexity of the actions takenby the Oracle RDBMS under a hot backup could consume an entire chapter and is beyond the scope of this book. What you should understand is thetrade-off for taking a hot backup is increased use of rollback segments, redo logs, archive logs, and internal buffer areas within the SGA.

--
Jeremiah Wilton http://www.wolfenet.com/~jeremiah Received on Wed Sep 23 1998 - 23:56:36 CDT

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