(Originally posted 2006-04-03.)
In this entry I talked about an APAR that fixed an issue with
mixed farms of control units, where only some supported MIDAWs.
Here’s another APAR. This one could increase the performance benefit of MIDAWs.
APAR OA15556 changes the way IDAWs (Indirect Data Access Words) are placed in memory, relative to the CCWs (Channel Command Words)…
(M)IDAWs point at the data for the I/O and CCWs point at chains of them. This enables
scatter/gather I/O processing from disparate ranges of real storage addresses. (There’s nothing new in this paragraph.)
The point of MIDAWs is that it enables more scatter/gather with each CCW and hence improves channel efficiency.
DB2 can pass up to 64 CIs (Control Intervals) to Media Manager. When it does the CCW and all the MIDAWs cannot fit in a single page. So you get 2 CCWs and hence less effective use of the (FICON) channel. NOTE:The second CCW cannot start until the first one has finished.
So APAR OA1556 splits the MIDAWs into a separate page from the CCW so it can all be done with one CCW, improving effectiveness./