TurboIMAGE/XL Database Management System Reference Manual (30391-90012)

354 Chapter7
Logging and Recovery
Roll-Back Recovery
Roll-Back Recovery
Roll-back recovery is another TurboIMAGE/XL recovery option. Roll-back recovery
provides rapid recovery of database data integrity following a "soft" system crash (that is,
system failure or loss of working memory). The roll-back feature is invoked through the
DBRECOV utility and requires only the current database log files to restore data integrity.
Note that ILR is not needed for roll-back recovery.
NOTE For roll-back recovery, the user log file and the database must be on the same
volume set when logging to disk. When logging to tape, the database and the
log file must be on the system volume set.
A database backup copy is not required for roll-back recovery. Regular backup of the
database is recommended, however, and is always required for roll-forward recovery in the
event of a more serious problem (for example, a disk head failure, or problems occurring
while roll-back recovery is in progress).
With roll-back enabled prior to a system failure, a record of each user transaction in the
sequence of occurrence is available to determine which transactions were incomplete at
the time of failure. When invoked, the roll-back recovery feature will "roll back," or undo,
any incomplete database transactions in the log file following a soft system crash.
NOTE A database must be enabled for roll-back before roll-back recovery can be
performed.
The following diagram illustrates the transactions of three different users at the time of a
system failure:
In the above illustration, the first user has completed one transaction (T1) and aborted
another (T4) prior to the system failure. Users two and three have each completed two
transactions, and each has one incomplete transaction at the time of failure. Individual
database transactions T1, T2, T3, T5, and T6 were completed and are properly reflected in
the database following system failure. Transactions T7 and T8, however, were incomplete
at the time of system failure, causing an incomplete modification of data to be reflected in
the database. These incomplete transactions (T7 and T8) are then rolled back (undone) to
their beginning, returning all affected data in the database to their original state before T7
and T8 began execution.