Description

   The SET FILE/RU_JOURNAL command marks an RMS file for recovery
   unit journaling. To use recovery unit journaling for a data file,
   a data file must be marked for recovery unit journaling with the
   SET FILE/RU_JOURNAL command, and transactions must be defined
   in an application program using DECdtm transaction services. You
   can also use this command to specify the default volume on which
   recovery unit journals will be created for this file.

   Use the SET FILE/NORU_JOURNAL command to unmark a file for
   recovery unit journaling. After you use the SET FILE/NORU_JOURNAL
   command for a file, modifications to that data file will no
   longer be written to a recovery unit journal.

   If you wish to delete a file that has been marked for recovery
   unit journaling, you must use the SET FILE/NORU_JOURNAL command
   before you can delete the file.

   There is no reason other than performance to keep recovery unit
   journals on a different volume from the file being journaled.
   Unlike after-image journaling, which protects against a system
   failure such as a head crash that causes a loss of data, recovery
   unit journaling ensures that a predefined set of operations are
   either done in their entirety, or not done at all. In the event
   of an abnormal termination of the application, such as a system
   crash or a Ctrl/Y, any incomplete transactions are automatically
   rolled back (undone). Because all recovery unit journals must
   be available before the data files can be rolled back, locating
   recovery unit journals on a volume where availability might be
   low could reduce the availability of the data files that use
   those recovery unit journals.

   Specifying a location for recovery unit journals for a file
   does not guarantee that the recovery unit journals will always
   be located on the named device or volume. For any active
   transaction, there is always only one recovery unit journal for
   local files. Thus, if many files are involved in a transaction, a
   single recovery unit journal is used, even if different locations
   for the journals had been specified (for individual files) with
   different SET FILE/RU_JOURNAL commands.

   Remote files are an exception to this rule. Each remote file
   associated with a transaction has its own recovery unit and
   recovery unit journal. The recovery unit journal resides on the
   remote system. The volume is chosen in the same way as for local
   files. Remote files have no effect in determining where the local
   recovery unit journal resides.

   A journal is not deleted when the transaction has been completed.
   Recovery unit journals are automatically deleted only when
   all of the files involved in the transaction are closed and
   the application exits. RMS journaling automatically creates a
   recovery unit journal at run time, whenever the first record
   stream associates with a transaction. All record streams in
   the process associated with the same transaction share a single
   recovery unit journal. Once a recovery unit journal is created,
   it can be reused for another transaction by the process that
   created it. A recovery unit journal is created only when there is
   no available recovery unit journal opened by the process for the
   current transaction.