Re: Are offline backups an option, or a neccessity?

Clay Davidson wrote:
>
> Oracle stores changed data in redo log files.
> These redo log files eventually update your database files.
> You can backup the redo logfiles and have a copy of every change made
> to your database since the LAST BACKUP of the datafiles.
>
> You can backup your datafiles by taking the system offline once a week
> or so and doing a cold backup. Reset your logfiles and start the
> process all over again.
>
> You can backup your datafiles while 'online' but what you are really
> doing is taking the datafile offline and backing it up. Any changes
> made to redo logs and your rollback segments (another subject) will
> need to be stored on disk until your datafile comes back online. Read
> mucho disk space if doing any heavy duty processes (read
> updates,deletes,inserts).
>
> If your database blows chunks at some point (which it will), you will
> need to restore it. If your last cold backup was months ago and you
> actually USE your database, then you will be restoring for several
> days (assuming you are not rotating your tapes, your tapes are good,
> there is something ON your tapes.....).
>
> Sounds like you paid alot of money to buy SAP then you paid probably
> 3-5 times as much for thier consultants and now you dont want to
> listen to them.
>
> Many sites 'MUST HAVE 24x7 AVAILABILITY' when they actually do not.
> Some sites will actually need it. They may lose millions of dollars
> whenever thier system is down. If you cant afford the disk space the
> consultant reccomended, then it is unlikely that you will lose much by
> beiing down for the length of time it takes to do the backup. The
> consultant may realize this and is giving you practical advice.
>
> My practical advice would be that if you do not trust the opinion of
> the person or company that you hired to do the job then you should
> postpone your implementation and replace them.
>
> On Tue, 18 Nov 1997 12:52:11 -0600, tashkop_at_iname.com wrote:
>
> >We are about to implement SAP over Oracle with a 24*7 requirement. I am
> >advised by the basis consultant of the implementation partner that we
> >should be taking the RDBMS offline at least once a week for an offline
> >backup, *or*, we need three times the amount of disk to carry out some
> >kind of complicated rolling type backup. This is in spite of the Oracle
> >RDBMS supporting an online backup mode, which we are advised to carry out
> >nightly.
> >
> >Is it really neccessary to take the system offline regularly to do a
> >backup? Can the online backups alone be relied on for a robust solution?
> >
> >I need to get my head around the issues here as the consultant said 'the
> >RDBMS supports an Online backup no problem' then in the next breath is
> >advising to take the system down regularly, or triple our disk budget.
> >
> >We would rather not do either if we can reliably not do so.
> >
> >your advice is appreciated.
> >
> >TIA
> >rgds
> >
> >
> >Peter Tashkoff <tashkop_at_iname.com>
> >Zespri International Limited Std Disclaimers Apply
> >All rights reserved. No party may use this document to vilify another.
> >
> >Zespri New Zealand Kiwifruit, The World's Finest
> >
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There is dangerous misinformation here!

Oracle hot backups DO NOT take datafiles offline.

Oracle hot backup places one or more tablespaces in 'backup mode' while you copy the underlying data files to disk or tape. In the meantime, Oracle continues to update all files just like normal.

How can this be? Oracle marks the file headers with the last checkpoint prior to the 'backup mode'. So, when you restore a file, the file header determines the log sequence to start recovery ( the control file knows the current log sequence).

If you lose a disk, you only restore the latest hot or cold backup of just that disk and roll forward using the logs (archived). If you take weekly hot backups, the worst case is to roll forward 6.99 days from the logs. If the system tablespace is ok, your database can be online during the recovery -- only the damaged files must be offline.

On another point, I have heard vast ammounts of drivel from highly paid consultants (SAP and others). If they do not know what they are talking about, replacement would be a good idea. Received on Sun Nov 23 1997 - 00:00:00 CST