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Re: Hot backup questions

From: Jeremiah Wilton <jeremiah_at_wolfenet.com>
Date: Tue, 26 Oct 1999 07:59:48 -0700
Message-ID: <3815C1E4.33714576@wolfenet.com>


Brian Peasland wrote:
>
> Jeremiah Wilton wrote:
> >
> > Tablespaces in backup mode are not out of sync, so they have no need to
> > resync when they are taken out of backup mode. Blocks are written to
> > the datafiles during backup mode as they are during normal operation.
>
> Is this true? Blocks are written to the datafiles during backup mode? If
> so, then the datafile is changed while it is being backed up resulting
> in an invalid backup of the datafile.

The backup datafile copies are inconsistent (not invalid), but then you apply logs to them during recovery to make them consistent.

> If memory serves me right, while a
> tablespace is in backup mode, any changes to it are written to the redo
> logs.

Changes written to tablespaces not in backup mode are also written to the redologs. Redologs record all changes.

> And the changes are written a block at a time. This means that the
> redo logs might fill up faster if there is high volume.

The logging of full blocks instead of just changed data for tablespaces in backup mode allows any split blocks (blocks read by the backup at the same time they were being written by dbw0) to be resolved during recovery by completely replacing the changed blocks in the datafiles with the good block images from the redologs.

> After the
> tablespace is taken out of backup mode, the changes are then applied to
> the datafiles.

That part doesn't happen. How come the end backup command returns instantly, even if the tablespace has been backing up for several hours? Over the years, so many people have not believed me, and asked for "proof" that datafiles are written during backup mode that I have prepared a repeatable proof that shows that Oracle writes data to datafiles in backup mode. It can be found at the following URL:

http://www.wolfenet.com/~jeremiah/hot-backup.html

--
Jeremiah Received on Tue Oct 26 1999 - 09:59:48 CDT

Original text of this message

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