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VMS Help RECOVER, Qualifiers, /BACKWARD *Conan The Librarian |
Rolls a file backward to a previous state. Use the /BACKWARD
qualifier to recover a file using a journal that has before-image
journal entries for the data file that you specify.
To specify a date and time to which the file is to be rolled
backward, use the /UNTIL qualifier. You should normally use this
qualifier for before-image recovery; if you do not use the /UNTIL
qualifier when you specify the /BACKWARD qualifier, the file is
automatically rolled back to the time when the first entry was
made in your before-image journal.
The file is rolled back using the before-image journal that was
specified when the file was marked for journaling with the SET
FILE/BI_JOURNAL command. You can override this default by using
the /JOURNAL qualifier.
You must always specify either the /BACKWARD or the /FORWARD
qualifier, but you cannot specify both.
When you use before-image recovery for a file that also uses
recovery unit journaling, the RMS Recovery Utility ensures
that the data consistency provided by recovery unit journaling
is maintained. The before-image journal includes records of a
transaction being started, prepared, committed, or aborted.
If you use the RECOVER/BACKWARD command to roll back a file that
is also marked for recovery unit journaling, the file is rolled
back and includes modifications for all transactions that were
completed (committed) on or before the ending time of the before-
image recovery, as follows:
o If, at the time before-image recovery is to end, there are no
active transactions, then before-image recovery takes place in
the same way as when there is no recovery unit journaling.
o If one or more transactions are active at the time before-
image recovery is to end, only those modifications made within
transactions that were completed (committed) on or before the
ending time are included in the file.
For example, suppose you issue the command RECOVER/BACKWARD
/UNTIL=10:30 for a file marked for both before-image and
recovery unit journaling. When before-image recovery is
complete, the rolled-back file will include all modifications
that were made within those transactions that completed on or
before 10:30.
o If you apply before-image recovery to a file that is used in a
multifile application, then you must use the /UNTIL qualifier
with the RECOVER/BACKWARD command in order to specify a
time to which the file is rolled back. You should then roll
back each of the files in the application to the same time
specified for the first file. If you do not use the same time
value for the /UNTIL qualifier, then the modifications for one
or more transactions may be restored to some of your files,
but not to others.
If you attempt to recover a file used in a multifile
application that uses more than one before-image journal,
the journals must be recovered in their entirety to assure
transaction consistency. If you use the /UNTIL qualifier, the
recovered files may still be inconsistent.
When the recovery operation is complete, the RMS Recovery Utility
displays the time of the last record it processed (rolled back).
This is generally the time of the first record modification
after the ending time (as specified with the /UNTIL qualifier);
however, it could be an earlier time if there were one or more
incomplete transactions at the ending time of the before-
image recovery. In this case, modifications made within such
transactions are automatically undone as part of the before-image
recovery operation, and the time of the last record processed
would reflect the first record modified within one of those
transactions.
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