HP 3000 Manuals HP 3000 Manuals
MPE/iX Transaction Management
When AUTODEFER is not enabled, TurboIMAGE/XL uses an internal MPE/iX
service, called Transaction Management (XM), to ensure physical
consistency of the database, to reduce physical disk I/O, to perform
Intrinsic Level Recovery (ILR), and to perform dynamic roll-back
recovery.
When XM is used, modifying intrinsics (DBPUTs, DBDELETEs, and DBUPDATEs)
are bracketed as XM transactions. XM logs these transactions in its own
XM log file. The volume set is the XM unit of logging and recovery.
Each volume set has one XM log file.
In default recovery mode, TurboIMAGE/XL uses XM to ensure the structural
integrity of the database. The writes of each database modification
(every DBPUT, DBUPDATE and DBDELETE) are bracketed as an XM transaction
and logged to XM log file pages. XM log file pages are written to disk
when one of the following occurs:
1. A system-specified time has elapsed.
2. A request is made by a subsystem, such as, TurboIMAGE/XL, or
another process to flush the log file to disk.
3. The XM buffer (or log file pages) is full.
Thus, intrinsics that have completed may not yet be written to disk. If
a system failure occurs, only completed modifications that have been
written to disk are recovered.
A dynamic transaction is denoted by the DBXBEGIN and DBXEND intrinsics.
With dynamic roll-back recovery, a dynamic transaction is rolled back
with a program abort or with a call to DBXUNDO. DBXUNDO can be called
when other database activity is occurring.
ILR is similar to TurboIMAGE/XL in that it also uses XM. When ILR is
enabled, changes to data sets are bracketed as XM transactions and logged
to XM log file pages. With ILR, however, XM log file pages are written
to disk at the end of each completed DBDELETE and DBPUT. If a system
failure occurs, only one DBPUT or DBDELETE will not be recovered. (A
completed DBUPDATE does not force a log write to disk.)
When AUTODEFER is enabled, MPE/iX Transaction Management is not used.
Instead, AUTODEFER uses the MPE/iX file system defaults which keep data
pages in memory until lack of memory or the closing of a file forces the
pages to be written to disk. In this mode, a system failure can cause
the loss of database integrity unless roll-forward recovery is used.