iX V6.5 (30391-90011)

352 Chapter7

Logging and 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.