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

Chapter 10 219
Technical Articles
Transaction Manager
Backdating the OS
While XM logger is doing copyfwd operation, it temporarily elevates
the version of the XM system log to a higher value and brings it back to
its original value once copy forward is over. But if the system crashes
while the XM syslog version is at a higher level, backdating it to an
earlier version of the OS wouldn't be possible. The system gives an
error and XM recovery fails. So in such situations, you need to complete
the recovery and then go for backdating the OS.
Checkpoint Improvement
Checkpoint utility is part of XM, and it is invoked when one-half of the
XM log is full. It writes out all the dirty data pages from that half to the
disk. During this activity, the system on-line response sometimes is
impacted due to the high priority nature of its disk I/O and also due to
the fact that you can checkpoint only one-half at a time. This makes the
logging slow down if the checkpoint process is slow or it holds up critical
system resources for too long. This becomes critical on high end MP
machines. This solution attempts to improve the performance of XM
checkpoint process so that on-line response becomes better and
transaction throughput increases.
This solution achieves an improvement in performance by not holding
one of the system-wide semaphores for too long while it's scanning the
entire virtual address range (could be 4 GB) of a modified file. It
remembers the dirty page ranges for each modified file in the log half
being checked and does posting to disk only for those ranges.
This solution affects only those files that are attached to XM user log,
typically IMAGE/SQL and KSAM files. It has no effect on system files
such as directories and label tables.
Enabling or Disabling It
The VOLUTIL tool is enhanced to provide two new commands to enable
or disable this solution for files on a given volume set. By default this
solution is disabled, which means the checkpoint process will scan the
entire file VA range for dirty pages.
The commands are as follows:
1. showchkptstat <volset-name>
This command shows the current state of the solution, that is,
whether it is enabled or disabled for the given volume set.
2. alterchkptstat <volset-name> ENABLE | DISABLE
This command enables or disables the solution for the given volume
set. If it's already in the given state, it will give a warning to that
effect.
You need SM capabilities to use these two commands.