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From: Mike Wilde 26-Apr-00 16:57
Subject: Database recovery from a hot backup


Database recovery from a hot backup

I just want to clarify a backup/recovery issue.

I have a hot backup procedure which performs the usual begin/end backup tablespace steps and backs up all the datafiles to tape.

If there are no archived logs created from the time the hot backup commences and when it finishes the procedure will not have any archived logs on tape. The current procedure does not backup the online redo logs to tape.

My question is, is it possible to recover the database if the building blows up if all I have on the tape are the datafiles and controlfiles, by creating a dummy, empty online redo log and forcing the database to recover as far as it can ? or will it fail without the online redo logs or at least 1 archived log ?

Thanks,

Mike

I am assuming you are running the database in archive log mode. Your recovery scenario would look like this - Copy back all the datafiles, control files, init/config files to the filesystem/directories where the previous database was.
Mount the database
Issue recover command - Oracle's OCI would read the controlfile for the tablespace names and the last checkpoint/SCN #'s and compare that with the actual datafiles, it will then determine where to start recovering from, it will ask you to supply the log pertaining to a particular log sequence #, if in archive log mode, it is expecting the file to be at the destination pointed to the ini parameter, when you confirm that the file it is looking for is in the spot where it is looking for, it will read the file and apply roll forward changes. If it cannot find the file, it will report, at which point you can issue a recovery cancel. After a recovery, Oracle will not allow you to open the database without resetting the logs, in which case the log sequence #'s will start over from 1. During the actual recovery however, the online redo logs are not used, only the archived logs are used. Your recovery is certainly challenged if you dont have enough archived logs - The archived logs are enough only if the duration and time frame of the saved archived logs covers the time frame when the hot backup was taken. There is also a catch with the control file, you have to start recovery with a control file in the future, one that was taken after the finish of the hot backup to enable the roll forward of the database. There is a strong reason why Oracle forces you to reset the logs.

It is not necessary that you have the online redo logs. Those are not used anyway until the database is opened. Even if the current ones are not available, it will create a group at the expected location provided the directory paths are accessible. Try this scenario -

You have 4 redolog groups. The one in use is group 2. Remove group 4, when it is time to switch redo groups, Oracle will automatically create the files in the OS. The idea of redolog multiplexing is to enable point in time recovery. In non-archive log mode, I have run into situations where a normal database startup has failed and I have had to clear log contents (basically lose info). Such scenarios have happened on startup after instance failures.

The recovery will not fail without having the online redo logs. You will be able to recover as much as possible with your hot backup data.

Kirsten Dimmick
Oracle/Midrange

Kirsten Dimmick
Oracle RMSC Midrange

Thanks for your replies, but I would still like to clarify:

Without online redo logs & the archive log that was created during the hot backup being taken, is it possible to recover the database with just the datafiles and a backup control file, or do the datafiles know that there is an archive log missing and therefore insist on it being there ?

Thanks,

Mike Wilde

Mike,
You will be able to do a incomplete recovery without all the files. You should be able to do a Recover database until cancel.
You will then have to reset logs to get the database to come open.
This would not be a suggested recovery plan unless you do not care how much data is lost.

Thank You
Mark Youngwirth