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

310 Chapter7
Logging and Recovery
Recovery Options
Recovery Options
The logging and recovery system is used to bring databases back to the same state at the
time of a system failure; this state does not include incomplete transactions.
TurboIMAGE/XL provides several types of recovery options:
• Default recovery mode
• Dynamic roll-back recovery
• Intrinsic Level Recovery (ILR)
• Roll-forward recovery
• Roll-back recovery
• Mirror database
The recovery options are described briefly below. Dynamic roll-back recovery, ILR,
roll-forward recovery, roll-back recovery, and the mirror database are discussed in detail
later in this chapter. A brief description of these recovery and logging options is found in
appendix G, "Recovery and Logging Quick Reference." Use the information in this chapter
and in appendix G to determine which recovery and logging options to choose. The
recovery option is based on available database backup, logging resources, and performance
requirements.
Default Recovery Mode. In default recovery mode, TurboIMAGE/XL uses an MPE/iX
file system service, Transaction Management (XM), to ensure the structural integrity of
the database following a system failure. All modifications to the database (DBPUTs,
DBDELETEs, and DBUPDATEs) are automatically logged to an MPE/iX XM log file. However,
this XM log file is only written to disk when one of the following situations is true:
• A system-specified time has elapsed.
• A request is made by a process to flush the log file to disk, for example, a call to DBEND
mode 2 or 4.
• The XM buffer is full.
XM ensures that the TurboIMAGE/XL intrinsics are applied to the log file in a serial
manner. If a system failure occurs, those completed intrinsics that have not been written to
disk are not recovered. Thus, one or more completed DBPUTs, DBDELETEs, and DBUPDATEs
can be lost, but the internal structure of the database remains consistent. Recovery with
DBRECOV must be performed at system startup time before anyone modifies the database.
Dynamic Roll-Back Recovery. Dynamic roll-back recovery is a method of recovery that
uses XM. XM ensures the physical and logical integrity of the databases. Dynamic
roll-back allows a more timely recovery of databases than is possible with DBRECOV.
Dynamic roll-back eliminates the overhead incurred when a database is enabled for user
logging and permits database access to continue, even when recovering a database. It
provides the most thorough protection for logical database transactions. Dynamic
transaction can span one or more databases. The dynamic transaction spanning multiple
databases is termed dynamic multiple database transaction, or DMDBX. Dynamic
roll-back handles both program aborts and system failures without downtime for recovery,