Re: [Networker] Why Oracle incremental backup is as large as a full?

2007-05-02 18:22:53
Subject: Re: [Networker] Why Oracle incremental backup is as large as a full?
From: Preston de Guise <enterprise.backup AT GMAIL DOT COM>
Date: Thu, 3 May 2007 08:23:18 +1000
Hi George,

> The NSR client resource specified the rman script as the saveset, and it
> had the appropriate
> nsrnmo script listed under the 'Backup command' field.
> The full rman script used a backup command like this:
> backup full filesperset 10 format 'FULL_%d_%U' database;
> The incremental rman script used a backup command like this:
> backup incremental level 1 cumulative filesperset 10 format
> 'INCR_CUM_%d_%U' database;
> We finally took the database off line. Later when re-testing, we changed
> the syntax of the
> two rman scripts respectively to this:
> backup as backupset incremental level 0 cumulative database;
> and:
> backup as backupset incremental level 1 cumulative database;
> We've not seen this behavior again.
> I realize it's impossible for anyone to really troubleshoot this, but
> can anyone offer any plausible
> explanations for this behavior, assuming, of course, that back when this
> was happening, we
> *really* were specifying the correct rman script and not the wrong one.

I remember reading, when I was first learning about RMAN backups with
NetWorker 5 or so years ago, that to be able to do incremental backups,
Oracle requires an Incremental-level-0 style backup rather than a full

>From what I can gather, the named full backup does not setup appropriate
"tags" (for want of a better term) in the database; the level 0, which is
still a full, does, and the level 1/etc that follows makes use of the
details setup by the level 0 backup.

I don't believe this is documented in the NetWorker docs though; from memory
I found it whilst digging through Oracle's multi-hundred page documentation
on backup and recovery.

I always followed their instructions, and always got proper
full+"incremental" backups. One thing of course is that while the
"incremental" is usually much smaller than the full, many of my customers
have found that it takes almost as long as the full, and is more intensive
on database processing than the full, due to the tablespace scans that have
to be done.

(But still, backing up 20GB of data rather than 2TB always brings a cheer to
the heart of a backup admin, so it's usually worth it...)




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