Re: Hot backup questions

I did some checking on this. I found it odd that the Oracle Unleashed (by SAMS Publishing) and the Oracle Backup & Recovery (Oracle Press) books both state that datafiles are not written to during hot backups. These were the sources of my previous comments. After Jeremiah's post, I read the article alluded to in his URL. I found it to be a well written article. Thanks Jeremiah! But there is always a danger of taking things from the web and citing them as truth. No offense intended here. So I did some more digging. In the Oracle Backup and Recovery documentation provided with Oracle 8.0.5, I found the following:

"A backup of an open database is inconsistent because portions of the database (hence, datafiles in the database) are being modified and written to disk while the backup is progressing. "

Thanks for clearing up my (and hopefully others) misconceptions about this.

Thanks!!!!
Brian

Jeremiah Wilton wrote:
>
> 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 Wed Oct 27 1999 - 08:30:22 CDT