There are two varieties of SYNC requests: immediate SYNC requests that must be completed as soon as possible and non-immediate SYNC requests that can be delayed, if necessary. If MVS™ determines that a non-immediate SYNC request will be delayed because of resource problems (for example, the subchannels are unavailable ), it will change the request to an ASYNC request. These requests are counted in the CHNGD field of the RMF™ CF Structure Activity report and their service times are included in the ASYNC service times. However, requests that are converted from SYNC to ASYNC only for heuristic reasons based on service time considerations, are not counted as CHNGD. Therefore, CHNGD can be used as an indicator of whether or not there are sufficient CF links or subchannels available to accommodate the workload.

As a rule of thumb, the total number of requests CHNGD should be less than 10% of the total requests to a given structure.

For more information, see How z/OS performs synchronous to asynchronous conversion of coupling facility requests.