6 Replies Latest reply: Mar 22, 2008 2:00 AM by 26741 RSS

    Multiplex redo log files?

    626474
      Dear all expert,

      I have a stupid question which is I manage the redo log using multiplex on different devices (disk a & disk b). If I lost all data, control & redo log files, I can perform "Complete Recovery" in order to get back data & control files from the backupset. For the current redo log files, I can copy redo log files from disk b if the files in disk b are working fine. How about I lost both redo log files in disk a & b? I only can issue "alter database open resetlogs" because I do not backup redo log files. Is it a normal operation to back up redo log files as well?

      FAN
        • 1. Re: Multiplex redo log files?
          ebrian
          NOPE...you should never backup redo logs. Here are your options for Recovering After the Loss of Online Redo Log Files.
          • 2. Re: Multiplex redo log files?
            285712
            Redo logs do not form part of the backup set. Unfortunately if you lose all redo logs your only option is to open resetlogs.

            regards
            • 3. Re: Multiplex redo log files?
              629704
              Never is a long time.

              While I agree with the generalization, if one is testing, and it might make sense to back everything up, including the online redo logs (assuming db is closed), so that later restores bring the db back to the exact same state.

              You'll want those backed up during a migration as well, unless logs have been switched and archived.

              Admittedly, these are very specialized and specific cases. I'd hate to see someone backup the online logs and then restore the db, with those log files, and lose data. That is the real danger.

              This is also one reason why some suggest online logs on raw - just to have another way to point out how special and different these files are.

              Chris
              • 4. Re: Multiplex redo log files?
                stevencallan
                Can you cite a reference anywhere in Oracle documentation that says to include online redo logs as part of a backup? If you can't, that is "never." And that is the case.
                • 5. Re: Multiplex redo log files?
                  629704
                  To clarify, and expand upon my earlier post, I agree that online redo logs should not be part of "backups".
                  • 6. Re: Multiplex redo log files?
                    26741
                    There's probably no documentation that "says to include online redo logs as part of a backup".

                    However, there are situations where you can (or even would want to) include your
                    redo logs in a backup. Note that I say "a backup", not "backups". A specific, adhoc,
                    backup may include redo logs.

                    1. I take a cold backup of a database and restore it to another server. I do not wish to do a RESETLOGS (why, I ask "do I HAVE to resetlogs ?"). Then, I include my redo logs in my cold backup and restore.

                    2. I am testing backup and recovery scenarios. I take a cold backup of the whole database as a "reference point" in all my various recovery tests.

                    3. I am attempting a recovery of a production database to verify data at different points in time. I know that I might have to also do a resetlogs in between, change the
                    incarnation id, maybe even change the db_name etc. I want to be able to revert to
                    any restored image during the few rounds of restore-and-verify-data. Those
                    intermediate backups would well be cold backups with redo logs (after resetlogs ?)

                    4. I am remotely advising a DBA on recovery of a crashed database. I cannot be
                    sure that he has verified all the archivelogs, got the current log sequence numbers etc.
                    I know that it is likely that one or more of the online redo logs hadn't yet been archived
                    and I would be able to "simulate" them as archived logs during the recovery -- but only
                    if I do ensure that the redo logs are backed up.

                    5. The last one or two archivelogs had not got included in the tape backup. However,
                    since I know that the database has 4 or 5 or 6 online redo logs, those last two are
                    still available "on-disk" in the form of "online" redo logs (which might very well be
                    raw devices, just to make things more complicated), which haven't been "lost" in the
                    filesystem crash.


                    Bottom Line : As a "general" rule, it is not necessary to backup online redo logs.
                    As a "general" piece of advise, it can be dangerous to blindly backup online redo
                    logs, even in cold backups.
                    However, there can be specific cases where I know I would want to backup the online
                    redo logs.

                    Remember that "online redo logs" doesn't mean exactly the same as "CURRENT"
                    or "ACTIVE".