Chapter 15. Global online change 233
15.9 Migration and fallback
You can have a mixed online change environment where some IMSs use global online
change and some use local online change. Of course, you would have to coordinate the
changes between the systems using global online change and those using local online
change.
Some members of the IMSplex be defined with global online change (OLC=GLOBAL in
DFSCGxxx), and the INITIATE OLC commands would effect all instances.
Some members could be defined to used local online change (the default or OLC=LOCAL in
DFSCGxxx), and would use the
/MODIFY commands on each IMS instance. You would then
have to coordinate the changes
IMS systems may be migrated to global online change one system at a time. The process is
shown here.
1. Define OLCSTAT data set
2. Run the DFSUOLC0 utility to initialize OLCSTAT data set, before the IMS is cold started
for the first time
3. Shut down an IMS
4. Remove MODSTAT DD statements from the IMS control region JCL
5. If you are using XRF, also remove MODSTAT2 DD
6. Define the IMS's DFSCGxxx with OLC=GLOBAL & OLCSTAT=dsname
7. Cold start the IMS
It is advisable to limit the time that some systems are using local online change and some are
using global online change. The support for the mixed environment is provided to facilitate the
migration of systems one at a time.
Fallback from global online change to local online change is also supported. This may also be
done one system at a time.
Define DFSCGxxx OLC=LOCAL
1. Shut down IMS
2. Add MODSTAT DD statement to IMS control region JCL
3. If you are using XRF, also add MODSTAT2 DD
4. Remove the OLCSTAT DD statement from the control Region JCL
5. Run INITMOD job to initialize MODSTAT data set
6. Cold start IMS
Important: Note that a change to Global online change requires a cold start of the IMS
system.
Important: Note that fallback to local online change from global also requires an IMS cold
start.

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