Re: rman compression

The Backup and Recovery Course not being quite so popular as it used to be (and can't you just tell from some of the posts here of late?!), I may be a bit out of touch with RMAN... but here goes for nothing...

RMAN doesn't "compress" backups, if by "compression" you're thinking of anything like WinZip or gzip.

Being an Oracle utility, however, it is intelligent enough to realise when a block on disk has never been used (ie, is above a high watermark somewhere), and will not include such blocks in the output backup sets. That contrasts, of course, with O/S backups which, not having a clue what they are actually copying, take a byte for byte copy of the physical datafile, unused blocks and all. Net result: RMAN backup sets are significantly smaller than a physical backup simply using O/S copy commands would be.

RMAN makes things even smaller when you do incremental level backups: again, because it understands Oracle blocks and data structures, it can tell whether a block has changed since the last backup... if it has, it gets included in the backup set. If not, it doesn't -and once again, such omission makes the resultant backup set much smaller than it otherwise would be.

If there is actually an RMAN command to compress a backup set a la WinZip, I'm not aware of it, and frankly I'd be gobsmacked ...Oracle generally takes the view that you play around with compression utilities at your peril. It's valuable data, after all.

Regards
HJR "Ted Chyn" <ericis6_at_home.com> wrote in message news:Bhlj7.139845$oh1.55982353_at_news2.rdc2.tx.home.com...
> sir, Can anybody shed light on how rman compress the datafile during
backup
> ?
> thnx ted chyn
>
>
Received on Thu Aug 30 2001 - 06:30:13 CDT