FICON Administrator's Guide v6.4.0 (53-1001771-01, June 2010)

FICON Administrator’s Guide 61

53-1001771-01

IU Pacing

• Blade swapping is not supported when swapping to a different model of blade or a different

port count. For example, you cannot swap an FC8-32 blade with an FC8-48 port blade.

• Blade swapping is not supported on application blades.

NOTE

This feature is not supported on the FX8-24 DCX Extension blade.

For more information on blade swapping, refer to the Fabric OS Administrator’s Guide.

IU Pacing

IU pacing is not, strictly speaking, a flow control mechanism, as flow control is adequately

addressed by the FC-PH level buffer-to-buffer crediting function. Rather, IU pacing is a mechanism

intended to prevent I/O operations that might introduce very large data transfers from

monopolizing access to Fibre Channel facilities by other concurrent I/O operations.

IU pacing provides a load-sharing or “fair-access” mechanism for multiple competing channel

programs. While this facility ensures more predictable I/O response times on heavily loaded

channels, it produces less optimal results for very-long-distance deployments. In these cases,

increased link latencies can introduce dormant periods on the channel and its WAN link. Dormant

periods occur when delays waiting for anticipated command responses increase to the point where

the pacing window prohibits the timely execution of CCWs that might otherwise be executed to

ensure optimal performance. The nominal IU pacing window for 1, 2, 4 and 8 Gbit/sec FICON

implementations permits no more than 16 IUs to remain uncredited. Pacing credits can be

adjusted dynamically from these values by control unit requests for specific protocol sequences;

however, the channel is not bound to honor control unit requests for larger IU pacing windows.

Frame pacing delay

Common FICON issues

Any information you need to verify that FICON has been set up correctly can be found in the Chapter

2, “Administering FICON Fabrics” or Chapter 3, “Configuring FICON CUP”.

Symptom Cascading: No path – Entry Link is “..”

Probable cause and recommended action

This typically occurs when one or more of the required high integrity features for cascading have

not been met. Check the following:

• Review each step in “Configuring switched point-to-point FICON” on page 16 and “Configuring

Cascaded FICON” on page 20 and validate that all features are configured properly.

• Validate with the data center’s system programmer that CUP is online for each director or

switch in the fabric.