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Backup and recovery questions

Posted on 2013-01-28
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Last Modified: 2013-01-29
What exactly happens behind the scenes (I understand that database restore process is started and that's not what I am asking) when you run

RESTORE DATABASE;

statement?

Does it retrieve the last backup whether it was full or incremental backup? Or does it only work for full backups?

Also what happens behind the scenes when you run

RECOVER DATABASE;
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Question by:YZlat
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9 Comments
 
LVL 77

Expert Comment

by:slightwv (䄆 Netminder)
ID: 38828147
>>Does it retrieve the last backup whether it was full or incremental backup?

In a nutshell: It does everything it needs to do to restore the database to the present.  It will use whatever resources it needs to perform the restore/recovery.

It may not even need a backup.  It may just need an online archived redo log file.

Depending on how detailed you want to dig, the online docs are probably the best place to start about what each of these commands does.
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LVL 35

Author Comment

by:YZlat
ID: 38828170
I have trouble with online documents and learn a lot better if someone explains it to me. If I could just read online docs, I would not post a question, would I?

I understand that it does everyhting it needs to but I was looking for a more fine-grained answer - how does it figure out which backup to use, etc.?
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LVL 35

Author Comment

by:YZlat
ID: 38828215
From what I read so far, looks like RESTORE command is not actually doing the resotre, but decided which files are to be used for the restore and verifies their contants. Is this correct?
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LVL 35

Author Comment

by:YZlat
ID: 38828241
Another thing I wanted to ask is does BACKUP DATABASE backs up the control files or do you need to do that separately?
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LVL 77

Expert Comment

by:slightwv (䄆 Netminder)
ID: 38828305
>>If I could just read online docs, I would not post a question, would I?

Unfortunately this site isn't really set up to be a training substitute.  It also doesn't lend itself well to very broad questions.   This is a very broad question.

>>how does it figure out which backup to use, etc.?

I believe everything is driven by the SCN.  It checks the current SCN in the control file then does some magic to figure out how far back it needs to go and if it has everything it needs.

I'm sure you are asking about the 'magic' that it does.  Sorry, but I don't know.

>>but decided which files are to be used for the restore and verifies their contants. Is this correct?

Sorry but I honestly don't know.  I've never really jumped into this end of the pool.  It is one of those commands that 'works' so I've never had a reason to ask 'how'.

Hopefully some other Experts will be aslong soon and might be able to help fill in some gaps.
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LVL 37

Accepted Solution

by:
Geert Gruwez earned 500 total points
ID: 38830027
in short:
RESTORE DATABASE;
> this only works if you have a started instance and restored the controlfiles first.
> checks what time your system is on: in this case: 29/01/2013 08:00:00
> looks in the controlfile for a list of available backups
>> in rman: list backups all;
> finds the first full backup before the current time
> restore all the datafiles

RECOVER DATABASE;
> checks current time
> finds what the restored database time is
>> oldest entry in v$recover_file
> checks all available backups after this time
> uses level 1 incremental backups to apply
> after this it uses archivelogs found in the backups

after restoring the controlfile, it's best to run a crosscheck backups with rman to verify backups are available

>>If I could just read online docs, I would not post a question, would I?
What's the difference between reading our comments and reading online docs ?
> the online docs are very extensive and explain almost everything
> we just give a summary here
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LVL 37

Expert Comment

by:Geert Gruwez
ID: 38830030
restore level 1 incremental did require a extra step but can't remember exactly what
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LVL 35

Author Comment

by:YZlat
ID: 38831764
>>What's the difference between reading our comments and reading online docs ?
For me the difference is pretty big and I only need a summary. I am learning too much at this time and I just would not be able to get details on everything and my life would be much more difficult without you guys and your "summaries":)


Another advantage is the ability to ask questions and clarifications:)

Sobasically RESTORE does not do the recovery but copies the files from backup and checks if everything is ready for the recovery? Is that correct?
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LVL 37

Expert Comment

by:Geert Gruwez
ID: 38833248
yes, restore only extracts the datafiles from the backup files
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