Re: Guaranteeing DB consistency with Split Mirror Online backups

Murali,

Thanks for the clarification about "alter database suspend" in Oracle 8i. I did not know that this command is in 8i. With using this command to suspend writes to the DB - what affect does this have on applications using the DB ? Do they hang until the resume, or are writes cached within the DB Buffer area in memory ? With this command used before the split takes place, I believe there is 100% assurance that the mirror can be split without any chance of loss of data - as long as the mirrors being split only contain Oracle data and metadata. In particular, my concern is at the UNIX filesystem level, writes are still occurring (to the Oracle redo logs, dbf files).
A split of the mirror while the writes are occurring could result in undetected data loss in the mirror copy of the files, as data may not be completely written (ie. a half-filled block, for instance). This would not be detected unless a dbverify is run on the entire mirror copy of the database, as the database may well start and recover itself without errors, and the filesystem may pass an fsck correctly - but data may be lost. Presumably, the synchronous nature of writing the redo log data will ensure recovery of the database only to the point of the last committed write - the question is when, during the backup process, did this last committed write take place ? Thanks for any further clarifications.... Ian.

"Muralidhar Prabhakaran" <muralip_at_earthlink.net> wrote in message news:<YEb68.10190$ks5.981785_at_newsread1.prod.itd.earthlink.net>...
> Ian,
> Let me answer your question in two folds. First oracle 8i does have IO
> suspend features. ALter database suspend does that.
> Secondly the split mirror backup can be accomplished by issuing the alter
> tablespace begin backup command just before the split and the alter
> tablespace end backup just after. By doing this the file headers are frozen
> and splitting at that point gives a backup although not consistent. But
> while recovering if we have all the archive and online log files then the
> recovery can be made consistent.
>
> HTH
> Murali
> "Ian Nash" <iannash_at_au1.ibm.com> wrote in message
> news:3bc3b5f4.0201302239.4115eb6f_at_posting.google.com...
> > Hi,
> >
> > I am trying to understand how Oracle provides a guaranteed consistent
> > database backup when using the "split-mirror" online backup approach.
> > Because the "split-mirror" relies on the underlying hardware and/or
> > software to manage, it is outside of Oracle's influence, and in
> > particular, is relying on the Operating System to manage the
> > consistency of the data in the filesystems in the mirrors.
> > In AIX, there is no guarantee about data consistency (ie all blocks
> > written and metadata consistent) unless the filesystem is unmounted.
> > This is why it is recommended that a filesystem is unmounted when a
> > mirror split takes place, as it ensures that all data blocks are
> > written, and that no changes to data blocks are taking place as the
> > split occurs.
> > Hence, my question is how does Oracle (especially Oracle 8i which -
> > AFAIK - does not have I/O suspend and resume features) guarantee a
> > consistent and complete DB backup using the split-mirror online backup
> > approach, when the underlying filesystem may not contain all written
> > blocks if a split is taken with the filesystem mounted ?
> > I have read the Oracle Whitepapers on this backup approach, and still
> > cannot see how data integrity can be guaranteed for the data copy
> > residing in the mirror.
> > Ian Nash
> > Adv(Thanks)ance
> >
Received on Thu Jan 31 2002 - 23:14:19 CST