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

Chapter 7 311
Logging and Recovery
Recovery Options
and your database will not be left with an incomplete transaction. Dynamic roll-back also
allows programs to selectively abort any ongoing transaction.
TurboIMAGE/XL uses XM to dynamically roll back uncommitted dynamic transactions.
Dynamic roll-back requires using the three intrinsics: DBXBEGIN, DBXEND, and DBXUNDO.
Dynamic roll-back recovery can be used with roll-forward recovery to handle disk media
failures.
Intrinsic Level Recovery (ILR). Intrinsic Level Recovery (ILR) is a recovery method
provided within TurboIMAGE/XL. ILR ensures that all completed DBPUTs and DBDELETEs
are recovered. On TurboIMAGE/XL, ILR is equivalent to calling DBEND mode 2 after every
intrinsic. It is recommended that ILR not be used. It is not required for the DBRECOV
roll-back recovery. See "Intrinsic Level Recovery" in this chapter for more information.
Roll-Forward Recovery. Roll-forward recovery is another method of recovery provided
within TurboIMAGE/XL. It is used to ensure the structural and logical integrity of the
database. Roll-forward recovery is used after a hard system failure such as a disk head
crash or after problems occurring while roll-back recovery is in process.
Roll-forward recovery requires user logging and a backup copy of the database. The
recovery time needed is generally more than roll-back recovery. The database backup may
have been done using DBSTORE, TurboSTORE/iX 7x24 (with ONLINE=START or
ONLINE=END option), or other similar programs and must be synchronized with the
current log file.
If DBSTORE was used for storing the database, DBRESTORE needs to be used to restore the
database. The correct log file is the first log file and recovery commences from the
beginning of the log file.
If TurboSTORE/iX 7x24 True-Online Backup was used (with ONLINE=START or
ONLINE=END option) to back up the database, the RESTORE command of MPE is used to
restore the database. The recovery may even commence from the middle of a log file in use
when backup was initiated. To find out the logfile from which recovery will start, use the
SHOW
databasename ALL command of DBUTIL. Refer to the TurboSTORE/iX True-Online
Backup discussion later in this chapter.
Roll-Back Recovery. Roll-back recovery is another method of recovery provided within
TurboIMAGE/XL. It is used to ensure the structural and logical integrity of the database.
Roll-back recovery is used after a "soft" system crash such as a system failure or loss of
working memory.
Roll-back recovery requires user logging. A backup copy of the database is recommended
(for protection in the event of a hard disk failure), but not required. The time taken to
perform roll-back is generally much less than roll-forward recovery.
Mirror Database. In addition to the recovery methods mentioned previously, roll-forward
recovery can be used to mirror a database for constant access or "high availability" while
providing controlled maintenance using a DBRECOV feature called STOP-RESTART.
Backups and down-time can be regulated with this maintenance method which consists of
two identical databases on two separate computer systems. The mirror database resides on
the secondary system and is maintained with user logging, DBRECOV, and periodic
DBSTOREs.