Communicator 3000 MPE/iX Release 6.0 (Platform Software Release C.60.00) (30216-90269)

216 Chapter 10
Technical Articles
Transaction Manager
Transaction Manager
by Nagarajan and Jyothi
Commercial Systems Division
Overview
Transaction Manager (XM) is an integral part of the MPE/iX file
system. It provides a facility to log all the transactions that are done on
a file that is attached to XM via a system intrinsic. It ensures data
integrity by protecting such files across system crashes.
Files on MPE/iX are of two categories: system files and user files.
System files are label tables, directories and SSM bitmaps. User files
are those created by the user. All the transactions on system files are
logged onto an XM system log file on the master volume of each volume
set, just as all the transactions on user files (such as IMAGE/SQL and
KSAM files) are logged onto an XM user log file on the master volume.
Both these XM log files are (magic) system files and are created at
volume set creation time with a fixed size.
A System Abort 2200 is generated from Transaction Management (XM)
subsystem when it cannot reliably continue logging data to the system
XM log file. The system XM log file is a 12Mb object allocated
contiguously on disk. XM divides this into two portions called log
halves, each being 6Mb in size. While data is being logged into one half,
the other half is being emptied. The emptying happens when the
transactions are committed and the log half is posted to disk. If we fill
one half and discover that the other half is not yet empty, XM will
generate SA 2200 in order to preserve the data still in the second log
half. This scheme ensures data integrity by not allowing uncommitted
data to be overwritten.
Typical long-running system transactions are those that involve
insertion of a file into a group/directory at the beginning, initializing file
contents at creation time, and others.
System Log Expansion
As the higher-end models of HP3000 become faster and faster, the
frequency of occurrence of SA 2200 has been going up. To alleviate the
severity of this problem, a new solution to expand the XM system log
per volume set is being provided. This will allow more logging to be
done before the log gets full.