Re: Crazy mistake in "Oracle Unleashed"

From: John P. Higgins <jh33378_at_deere.com>
Date: Wed, 30 Sep 1998 14:38:26 -0500
Message-ID: <361288B2.E29850F2@deere.com>

Thanks, John Durst!

That was exactly my point.

Think about the backup of a large tablespace (several 2GB files) while updating. Several REDO logs may be created (and archived (and overwritten) ) during the backup. How could Oracle read the REDO logs at then end of the backup?

John Durst wrote:

While putting a tablespace in backup mode does alter the generation of redo to
include entire blocks for that tablespace, that doesn't mean the DBWR process
stops writing to the datafiles. The datafile header is frozen at the begin backup
so that at recovery time (if media recovery is necessary) we know where to begin
in the redo log history. Entire blocks are written in case the DBWR writes into
the same block the backup process is reading. The end backup command just updates
the datafile header when the backup process is complete. If you reread your own
post you will see that only the datafile header blocks are not updated during the
online backup.
satar@my-dejanews.com wrote:
> John,
> read my post...Modified blocks are written to the REDO LOGS. After the END
> BACKUP is issued and at the next log switch, the changed blocks are then
> updated in the datafiles. It is such an easy concept to grasp, yet so many
> DBA's are confused about this procedure. Maybe I'm wrong, but I would hate to
> believe that I paid $1400 dollars to Oracle Education to mislead me on Backup
> and Recovery.
> Satar Naghshineh
> In article <36115B13.89938F9D@deere.com>,
> "John P. Higgins" <jh33378@deere.com> wrote:
> > <HTML>
> > It is true: Oracle stops updating file headers durring backup mode.
> >
> > It is also true: Oracle writes changed blocks to the files durring backup
> > mode.
> >
> > satar@my-dejanews.com wrote:
> > <BLOCKQUOTE TYPE=CITE>OK, I think everyone is confused. According to
> "Oracle7:Backup
> > and Recovery
> > Volume Two * Student Guide" which I obtained durring my DBA Master's
> > program
> > held by Oracle Corporation, on page 10-13 through page 10-17 it tells
> > you the
> > following: After you administer BEGIN BACKUP...This action suspends
> > updates
> > to the header block of each database file belonging to the Tablespace.
> > If a
> > log switch occures, the sequence number information is updated as shown.
> > A
> > record is also written to the active redo log file to indicate that
> > an online
> > backup has begun. END BACKUP...This action will cause the header blocks
> > for
> > the datafiles belonging to the tablespace to be updated at the next
> > log
> > switch. If a log switch occures, the sequence number information is
> > updated
> > as shown. * The time between the ALTER BEGIN and ALTER END commands
> > should be
> > minimized, as more redo information is generated and written to the
> > redo log
> > files durring the backup since modified blocks are written to the redo
> > log
> > files. * Prior to Oracle7 release 7.2, any instance faliur while an
> > online
> > backup was in progress could cause the subsequent automatic recovery
> > at
> > startup to fail due to the header on the backed up file being frozen.
> > Oracle7
> > at startup would then detect that the header was out of sync with the
> > rest of
> > the files, and startup would fail with the message the file needed
> > recovery.
> > New to release 7.2, you can allow an online backup to be ended before
> > opening
> > the database after a system failure, by executing the ALTER DATABASE
> > DATAFILE
> > END BACKUP command.
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Received on Wed Sep 30 1998 - 14:38:26 CDT