Re: Oracle/VMS Backup

In article <768907062snz_at_hiredata.demon.co.uk>, <Parker_at_hiredata.demon.co.uk> writes:
> I am trying to design a backup strategy for an Oracle7 database running
> on VAX/OpenVMS. I am experienced with VMS but new to Oracle 7. I hope
> someone out there can help me. I have read the very useful Oracle Backup
> FAQ [...]
>
> 1) If I preserve the archived redo logs, do I need to worry about
> taking more frequent backups of the database?

No, you need to preserve the archived logs between full backups of the (shut down) database. the logs allow you to roll the latest full backup forward to the moment of a disaster. Therefore, plan to make a full backup often enough to avoid retaining huge amounts of archived logs.
>
> 2) Using BEGIN & END BACKUP:
>
> a) what does Oracle do in this mode?

it avoids writing to the tablespace during backup mode. all changes are posted to the redo logs and the rollback segments during this time. when backup mode is removed, the database writer updates the tablespace. this explains why it's safe to backup a file in backup mode: no writing is performed.
>
> b) surely the backups will not be consistent if I backup tablespaces
> individually as recommended by the Backup FAQ?

you do not have to worry about it PROVIDED you complete backing up your entire set of tablespaces and archive logs. when you recover to a point-in-time, all the archive logs will be scanned to roll the files forward to that time. it doesn't matter when the backups were made, nor when the archives were saved. consistency is guaranteed PROVIDED you remember to save all the tablespaces and all the archive logs.
>
> c) are there any restrictions on user operations while in this mode?

backup mode is transparent to the average dumb user. the problem that can ensue is when a wise guy reloads his entire 4-gigabyte table while you are backing up his tablespace. the risk is that the re-do logs will fill up before the writer can flush the changes to the tablespace. at that point his transaction hangs. of course the rollback segments will have hit the extent limit by that time and his transaction will roll back :-)
>
> d) The FAQ warns against putting all tablespaces into BEGIN BACKUP
> at the same time due to problems restoring if the system crashes.
> What are these problems?

i see it as a problem of strangling the dbwriter, almost asking the re-do logs to fill up and freeze the entire database. i imagine you could recover after a crash during this time, but you might lose transactions.

everybody has their own opinions about safety during backups, these are mine.

--
dave mausner _at_ braun technology group @ chicago.