When structures are used for the z/OS System Logstreams, the VSAM differ uses a throttling technique to avoid offloads and keep data in the structure until it is published and consumed by the downstream Engine(s). This provides better performance and more efficient use of CPU instead of having the publisher retrieve data from offloaded datasets. Throttling is controlled by the specification of High and Low Offload percentages for structure utilization. It is important to size structures to avoid throttling if possible while at the same time to avoid offloads.
For VSAM Differ, the SQDDFCDC program expects to read its input from //SQDPARM DD and throttling is turned ON by default.
//SQDPARMS DD *
--ptk=14
/*Throttling slows down the LogStream (zlog) put process used by the Capture to give publishers time to catch up and hopefully avoid offload. It is a crude control, and sometimes not very effective because it can cause the capture to slow down by elongating the runtime of the batch process. The Capture and Publisher do not communicate with each other to refine it. Some customers do however choose to disable throttling with the PTK because they would rather have offloads occur than slow down change data capture.