Backup Question

[Kutty]

Hi all,

While we take hot backup of tablespace we write

alter tablespace begin backup;

and lastly alter tablespace end backup;

What exactly these two commands perform? If suppose the backup of the above tablespace takes 1 hour and by that time if oracle writes data into that tablespace datafile what will happen? What role above 2 commands will play?

Thanks

Kutty

[sazzadur]

Hi,

when you issue alter tablespace begin backup oracle does a checkpoint in order to write dirty buffers to disk and then freezes the headers for that tablespace. The tablespaces is still available to users and all redo generated is written to the redo logs.
Redo generated whilst a tablespace is in backup mode consists of actual row data not just the rowid.

Once you issue alter tablespace end backup, oracle in effect does a recovery of the tablespace by applying all relevant redo logs in orde to sysnc the whole dtabase.


Hope this helps

Shaz

[Sonia]

hi

Oracle needs to maintain the datafile headers in a consistent state while they are being backed up. While u issue BEGIN BACKUP oracle stops updating the checkpoint in the file headers of the target datafiles.

During this period all the changes are recorded in the redolog file.


regards
sonia

[sazzadur]

You should do a

alter system checkpoint once you have taken the tablespace out of backup mode so that the databse is synched immediatly rather than waiting for oracle.

[Sonia]

hi,

i think it is not necessary..... of course there is no harm in doing that.

oracle does it implicity after a hot backup is completed.

Oracle advances the file header to the current database checkpoint, once u issue ALTER TABLESPACE END BACKUP statement to take the tablespace out of hot backup mode.