Re: Hot Backup Question

QAFT97A_at_prodigy.com (Ernst Renner) wrote:

>Hello All!
>
>I am performing hot backups on an HP-UX system. After putting a
>tablespace in backup mode, I copy the datafiles over to a near-line
>device. At this time I perform a check sum of the two files to ensure
>the same size and gain a sense of comfort o f the backup status.
>Now, it was my understanding that when a tablespace is in backup that
>transactions are NOT written to the physical file until the tablespace is
>taken out of backup mode. When I do my check summing, on the more
>volatile tablespaces, the check fails because it looks like the file has
>changed since I copied it.
>Does anyone have any idea on what to check, if my theory is incorrect, or
>basically any idea on this topic? This has just started recently, as my
>OLTP users have increased their online durations to 24/7. I had had
>about 5 hrs/night to do this where there were no transactions ongoing or
>outstanding.

It's my understanding that changes still get written to the online data files belonging to the tablespace being backed up. Only the headers of those datafiles get frozen for the duration of the backup.

If you need to recover using the backup copies, changes applied to those files after the SCN in the headers are applied using the archived redo logs during database recovery, bringing them back up to date. Therefor you *must* be running in archivelog mode to use hot backups.

Check out page 24-4 of the Concepts manual for a good explanation under "Online Data File Backups".

--
Chuck Hamilton
chuckh_at_dvol.com

Never share a foxhole with anyone braver than yourself!