Here is an interesting problem on hand that I need some insight into. We had a 2Gb MainFrame 2064 Ficon channel reading data from a 4Gb disk. The mapping was 1:1. The data was all SAP. There was a IBM 2109-M48 Ficon director in between with its port at auto-auto and default Buffer2Buffer credits set to 8. We experienced a huge performance dip. IBM SE's increased the B2B credits to 26 on all ports connected to the channel and disk and the problem went away. At first glance this clearly looks like a problem with the switch buffers. But there are some details that need to be looked into before one can say for sure the switch is a problem. Was the channel unable to process the data on time and hence could not release the buffers back to teh switch causing a backlog ? Was this anything to do with the type of data (SAP) that was being read ? Could getting the disk to speak at 2Gb have helped the situation ? What really happens when once increases the B2B credits dynamically ? Could this action have cleared any congestion the switch was experiencing ? Would I have seen this problem if I had a MDS instead ? If not, why ?