System Failure Recovery Options

DBRECOV Rollforward Recovery

Requirements

Logging must be turned on.

Method

After a system crash, the most recent backup copy of the database is DBRESTORE’d to disc. The log file is then reapplied to the database to bring it up to date.

Drawbacks

DBRECOV is time-consuming and costly, since it keeps users away from productive processing.

DBRECOV uses a technique of recovery called "staging," whereby the restored database is updated from the log file via staging files. Because large numbers of transactions can be ignored if an "end" is not found, a great deal of time and effort can be spent on forward recovery with no guarantee the recovery will be complete.

NOTE Rollforward recovery cannot be used to recover from a program abort.

Intrinsic Level Recovery (ILR)

Requirements

ILR must be enabled.

Method

An individual intrinsic, such as DBPUT to detail a dataset, may require the setup of several links or chains to the associated master datasets. If the system fails during the setup of one of these links, the result will be partially completed intrinsic with broken chains. With ILR enabled, each chain-modifying intrinsic is written to an ILR log file before the intrinsic is attempted. If the intrinsic is not completed, the information in the ILR log file is then used to complete the intrinsic (for TurboIMAGE) or to remove the intrinsic (for IMAGE).

Drawbacks

ILR adds considerable overhead to the system, because at least two additional I/O’s are required for each DBPUT or DBDELETE.

TurboIMAGE Rollback Recovery

Requirements

Both logging and ILR must be enabled.

Method

TurboIMAGE allows you to forward recover with DBRECOV as does TurboIMAGE, but it also allows you to initiate a rollback recovery. Rollback recovery eliminates the need for a DBRESTORE and for reapplication of logged transactions to the database. It allows you to roll out any partial transactions from the current database, leaving complete transactions in place.

Drawbacks

Having both logging and ILR enabled can have a considerable impact on system performance. As with rollforward recovery, productive processing must be stopped while incomplete transactions are backed out. In addition, rollback recovery is ineffective when a program abort logically damages a database.