Re: Oracle backups

On Fri, 11 Sep 1998, Frank Lagorio wrote:  

> gresmi_at_aent.com wrote in message <6t462k$ge2$1_at_nnrp1.dejanews.com>...
>
> >downloaded the freeware RMAN from Oracle. Oracle is running in archivelog
> >mode. We are very interested in the possibility of doing hot backups daily
> >during the week and a full cold backup on the weekend.
>
> Hot backups mean you offline a datafile and copy it, then online it and
> offline the next datafile and copy it until all datafiles are copied.
> Archived log files capture the input during this process.

This is not exactly true. When you perform an Oracle7-style hot backup in Oracle, the datafiles remain online and writable. For each tablespace placed in backup mode, the datafile headers are marked as such, and Oracle begins logging full block images to the redologs for that tablespace. The datafile are absolutely *not* offlined in this scenario.

In the event that a recovery later becomes necessary, Oracle is able to correct any split blocks (database blocks that were written by Oracle while the O/S was reading them, causing the O/S to read an inconsistent block) by applying the block image in redo back to the datafile wholesale.

The great thing about hot backups is you can back up a whole database or just some files. If you are using ARCHIVELOG mode there id *no* reason to do cold backups. There is no danger or exposure involved in performing hot backups. It simply changes the approach.

With the advent or RMAN, Oracle has finally provided a data-block-aware mechanism for reading datafiles for backup. With RMAN, you can perform online backups without switching tablespaces in/out of backup mode. Since Oracle is coordinating the read, it guarantees it doesn't feed you any split blocks. RMAN even does incrementals, allowing you to back up only the changed blocks in a datafile.

Storage managers like Legato Networker and Veritas NetBackup have provided products that work with RMAN. Unfortunately, I don't know anything about OmniBack, so I can't tell you if they have a product that works with RMAN.

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Jeremiah Wilton http://www.wolfenet.com/~jeremiah Received on Mon Sep 14 1998 - 18:16:48 CDT