Re: Hot Backup Question

"Chucky" <chuck.carson_at_syrrx.com> wrote in message news:3DFF7D04.6070408_at_syrrx.com...
>
> I recent book was reading did an extra step Ihave never done before when
> performing hot backups. It temporarily disabled archive logging while it
> copied the archivelog files to the backup destination. Won't this lose
> transactions that occur during that window?
>

No, but it risks hanging the database. New transactions still generate their redo in the Log Buffer, and LGWR still transfers those successfully down to the online redo logs. By disabling archiving, you simply stop ARCH from copying the online logs to archives.

The reasoning, I suppose, is that without doing this, were you to blindly just copy everything in your archive log destination to backup, there is a risk that you would be copying an archive log that ARCH is still writing to. A hot copy of an archive is going to be a mess, and you'll effectively have backed up a pile of poo in the middle of your redo stream.

By switching ARCH off, you guarantee that nothing is hot, and the backup is therefore not at risk.

It is, of course, a completely idiotic thing to do, for two main reasons.

Firstly, the risk of the database hanging (albeit temporarily). Although you've switched off ARCH, your database remains in archivelog mode. That mode means 'don't over-write an online log unless it has been copied'. You've just switched off the one thing that does the copying. Therefore, if LGWR switches through all available online logs, it will loop back to the first one waiting to be copied by ARCH, and be unable to proceed. Result: nobody can start new transactions.

Secondly, it is completely unnecessary. People do this, I suppose, in the mistaken belief that their backup will be as 'complete and up-to-date' as it's possible to get, and that this must be a good thing because it minimises the chance of data loss. What they always seem to fail to grasp is that backup is an on-going process, not a one-off event, and that what they fail to back up today will be backed up tomorrow. That it is therefore perfectly OK to 'miss out' the last archive log from today's backup, because you'll catch it again next time. That no Oracle database ever need lose a single committed transaction, however you backup, provided you maintain a continuous and uninterrupted stream of redo. And that there is therefore nothing to be gained by making a particular backup 'as complete as possible'.

So, yes it is possible to do it. No, there's no need to do it. And yes, it's a toitally moronic procedure to include in one's regular backup procedures.

Regards
HJR
> Thx,
> CC
>
>
>
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=----- Received on Tue Dec 17 2002 - 13:47:30 CST