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Dynamic Coupling Facility Dispatching (DCFD)
With DCFD, the coupling facility uses CP resources in a shared CP environment
efficiently, making a coupling facility using shared CPs an attractive option as a
back-up coupling facility. Without DCFD, a coupling facility using shared CPs would
attempt to get all the CP resource it could even when there was no real work for it
to do. With dynamic coupling facility dispatching, the coupling facility monitors the
request rate that is driving it and adjust its usage of CP resource accordingly. If the
request rate becomes high enough, the coupling facility reverts back to its original
dispatching algorithm, constantly looking for new work. When the request rate
lowers, the coupling facility again becomes more judicious in its use of CP
resource.
This behavior plays well into a hot-standby back-up coupling facility role. In back-up
mode, the coupling facility will have a very low request rate so it will throttle back to
very low CP usage. In this mode, the requests themselves will experience some
elongation in response time but this will not adversely affect the performance of the
overall system. Since the coupling facility is not consuming more CP resource than
it needs to, you can now set the processor weights for the coupling facility to a
value high enough to handle the load if the coupling facility was to take over for a
failing primary coupling facility. If the primary coupling facility does fail, the requests
can be moved immediately to the back-up coupling facility which can then get the
CP resource it needs automatically with properly defined LP weights.
Dynamic coupling facility dispatching is particularly useful in configurations where
less than one CP of capacity is needed for the back-up coupling facility use.
Dynamic coupling facility dispatching is automatically enabled for any coupling
facility LP that uses shared CPs, except for stand-alone coupling facilities. To
enable dynamic coupling facility dispatching on stand-alone coupling facilities, use
the DYNDISP coupling facility control code command. See "Coupling Facility
Control Code Commands" on page 2-50. It is recommended that anytime a shared
processor is used for a coupling facility, whether standalone or ICF processors, that
DCFD be enabled by insuring that the DYNDISP coupling facility control command
be set to ON.
Both the ICF and dynamic coupling facility dispatching enhancements represent real
production configuration options at significantly reduced cost. You will need to
examine the RAS trade-offs for each potential configuration to determine their
acceptability for your environment.
Dynamic Internal Coupling Facility (ICF) Expansion
Dynamic internal coupling facility (ICF) expansion allows an ICF LP to acquire
additional processing power from the LPAR pool of shared general purpose CPs
being used to run production and/or test work on the system.
This is very useful when you are using an ICF LP to back up another coupling
facility. In this scenario, the ICF, using dynamic ICF expansion, can acquire
additional processing capacity to handle the full coupling facility workload.
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PR/SM Planning Guide
Important Note
Dynamic ICF Expansion is not recommended for coupling facilities that are
used for primary production workload. All processors assigned to the coupling
facility partition should be dedicated to that logical partition if it is used used
for primary production workload.
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