Re: online backup

OK, I looked that section in the book over again and I was wrong. Changed blocks in a tablespace being backed up are logged to the redo log (if _LOG_BLOCKS_DURING_BACKUP is set to true, the default) but they are also written to the datafile as well. Queries will be satified from the datafile if the blocks aren't cached. This is why the backup copy of the datafile may contain "split blocks" that are inconsistent. These have to be resolved from the redo/archive logs should a restore be necessary.

St Erroneous wrote:

> jhy <jhy_at_earthling.net> seems to have written:
> >I have the same book. The reason the checkpoint SCN is not updated in
> >the file header(s) for a tablespace being backed up is because no
> >updates have been logged to the file since the backup started.
>
> [deletia]
>
> This has to be wrong.
>
> What would happen when a query attemped to drag data out of the file
> that had been updated since the hot-backup began? The block buffer can't
> be guaranteed to hold all changes made during a hot backup, and the db
> can't go around dragging data out of the online/archived redo logs to
> satisfy a query.
>
> It's my belief that during a hot backup, updates to the datafile
> continue as normal - ie, the product of your backup operation is
> inconsistent to a point in time or SCN, because your backup operation is
> reading data that's being changed by dbwr on the fly. The datafile
> itself remains current - to satisfy queries - but your OS copy of it is
> inconsistent.
>
> The redo logs contain hugely more information than they do for normal
> operation because instead of containing the _changes_ to be made to each
> data block, they contain the full image of each block after each change
> has been applied.
>
> This is because the DB has no guarantee that your OS backup of the
> datafile has the "before image" of the data to which it might apply a
> change made during the enable-backup phase. Thus, it needs to replace
> the whole of each altered block during recovery to attain a clean
> datafile image, consistent to the very end of the hot backup operation.
>
> Ie, your hot backup is useless without the archived redo logs from the
> the period of the hot backup. But we knew that already.
>
> Or something like that.
>
> Some or all of the above is probably wrong, but it's late and my brain
> hurts.
>
> -michael
> --
> St michael (mainly) Erroneous http://goliath.mersinet.co.uk/~ishamael/
> "The Lord's my backup, I shall not DAT.
> He maketh me pay cash-in-hand for DLT jukeboxen.
> He leadeth me in the Legato Networker restore."
Received on Sun Aug 16 1998 - 11:37:53 CDT