TurboIMAGE/XL Database Management System Reference Manual MPE/iX V6.5 (30391-90011)

354 Chapter7
Logging and Recovery
Roll-Back Recovery
words for the current time stamp). The roll-back time stamp is updated and logged in the
log file and in the root file when the database is first opened. Roll-back recovery then uses
the time stamp during recovery to verify the correct log file for each database.
Note that if the database and the log file are not on the same volume set, an error is issued.
CAUTION In the event of a system failure, do not restart user logging before running
DBRECOV. Log records may have been lost due to the system failure. If user
logging is resumed without a recovery, the resulting discontinuous log file
would cause invalid results in the event of a subsequent recovery. To prevent
access to the database after a system failure without recovery, enable the
MUSTRECOVER feature when you enable the roll-back feature.
Disabling the Roll-Back Feature
To disable the roll-back feature, make sure that no users are accessing the database
involved. Disable the roll-back feature by entering the following DBUTIL command string:
>>DISABLE database name FOR ROLLBACK
When roll-back is disabled, DBUTIL resets the roll-back flag and roll-back time stamp, but
not the logging flag; therefore, logging is still enabled.
CAUTION DO NOT DISABLE ROLL-BACK IF ROLL-BACK RECOVERY MUST BE
USED LATER. Disabling roll-back resets the logging time stamp. After the
logging time stamp is reset, roll-back recovery cannot be performed with the
current log file on the named database. The data in the database is
considered correct and therefore cannot be rolled back.
When the >>DISABLE command is issued, DBUTIL prompts a warning to remind you that
the time stamp will be erased and prompts for a response as follows:
WARNING: ROLLBACK time stamp will be erased.
Please type Y to confirm your disable command>>
If you enter Y, DBUTIL continues to disable roll-back. If Y is not entered, the >>DISABLE
command is not performed.
Performing Roll-Back Recovery
To complete the transaction roll-back process following a system failure, perform the
following steps:
1. Following a start recovery operation to bring up the system, locate the applicable log
file media to be used for transaction roll-back. If logging to tape, the correct tape needs
to be mounted. If using the MPE/iX CHANGELOG feature and there are multiple log file
tapes, this will be the first tape of the series. If logging to disk, TurboIMAGE/XL