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Home -> Community -> Usenet -> c.d.o.server -> Re: Crazy mistake in "Oracle Unleashed"
A copy of this was sent to Jeremiah Wilton <jeremiah_at_wolfenet.com>
(if that email address didn't require changing)
On Wed, 23 Sep 1998 21:56:36 -0700, you wrote:
>
>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.
>
You are correct that they are wrong...
<quote>
If you are in a situation where your database must be up and running 24 hours
a day, 7 days a week, you will need to perform inconsistent whole database
backups. A backup of an open database is inconsistent because portions of the
database (hence, datafiles in the database) are being modified and written to
disk while the backup is progressing. The database must be in ARCHIVELOG
mode to be able to perform open backups.
</quote>
The datafiles are written to during a hot backup for sure.
The datafiles that are backed up are 'fuzzy' or inconsistent -- when you restore them, they will have the equivalent of a point in time recovery applied to them -- this does not take place AFTER the end backup but only when/if the datafile is recovered. This is the stage where the archived and online redo log files are used to bring the restored datafile upto a consistent -- non fuzzy state.
The begin/end backup are need to tell us that the datafile(s) being backed up is being backed up (so when we restore it, we know the last checkpoint that completed for that file(s) and we know what redo we need to apply to recover it) and to tell us to store more information in the redo log files to make recovery possible.
>*****************
>From Oracle Unleashed 2nd Ed. (SAMS Pub.)
>ISBN: 0672311488
>
>********** begin citation/
>
>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.
>
>***************** end citation/
Thomas Kyte
tkyte_at_us.oracle.com
Oracle Government
Herndon VA
--
http://govt.us.oracle.com/ -- downloadable utilities
Anti-Anti Spam Msg: if you want an answer emailed to you, you have to make it easy to get email to you. Any bounced email will be treated the same way i treat SPAM-- I delete it. Received on Thu Sep 24 1998 - 08:21:30 CDT
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