Re: Hotbackup

From: Howard J. Rogers <dba_at_hjrdba.com>
Date: Sun, 14 Apr 2002 20:58:40 +1000
Message-ID: <a9bnfn$9d4$1@lust.ihug.co.nz>

Titi wrote:

> Hi,
>
> I do my backup whith:
>

>>alter tablespace toto begin backup
>>host cp ...
>>alter tablespace toto end backup

> For all tablespaces
> I am in archivelog ( 8.1.7 )
>
> Is it a right way to do the backup ?
>
> What RMAN do more ?
>
> Thanks ...

It's rather the case of what RMAN do less. When you perform a backup as you you have described (which is perfectly correct, by the way), Oracle has to prevent block fracturing by generating entire block images of redo the first time a block is modified by a piece of DML instead of just the normal few-bytes-sized-redo that the DML would ordinarily generate. That means the volume of redo generated increases by a factor of 10, 20 or even 30 times. When RMAN is used to perform the backup, no increase in the amount of redo generated takes place, because RMAN uses an entirely different mechanism to prevent block fracturing.

Net result: RMAN impacts the system much less than the sort of O/S backup technique you have described, and is thoroughly recommended as a result.

A full explanation of what the 'begin backup' command does in the system is available from my website http://www.hjrdba.com under the "Tips", "Backup and Recovery" links.

Regards
HJR Received on Sun Apr 14 2002 - 05:58:40 CDT