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It took just a little longer than expected, but here is Part 2 of our series examining how SyncSort for z/OS exploits the advanced facilities of the z/OS operating system and zSeries computers. In the first article we covered PAV technology, this time we turn to MIDAW.

What Is MIDAW?

The Modified Indirect Address Word facility is a modification to a channel programming technique that has existed for decades. An Indirect Address Word (IDAW) contains I/O addresses which are used by channel programs. The use of IDAWs is tailored to movement of records to or from contiguous buffer areas. With more complex channel programs, such as those which use data chaining to read or write physical blocks of data to or from non-contiguous buffer areas, CCW/IDAW structures are not as efficient, especially in today’s I/O environments. Complex channel programs can generate significant amounts of CCW and control information traffic on the channels, thus reducing the amount of bandwidth available for transmitting data.

Figure 1: Channel activity without MIDAW

The Modified Indirect Address Word was created to address these inefficiencies. When MIDAWs are used, the amount of CCW and control information traffic can be significantly reduced, allowing more data to be transferred. This enhances the performance of a job’s I/O processing, leaving bandwidth for other applications to use.

Figure 2: Channel activity with MIDAW

The use of MIDAWs makes FICON channels substantially more efficient at processing certain types of channel programs. DB2, Media Manager and PDS index processing are some of the IBM programs that benefit from the MIDAW facility.

How Does SyncSort for z/OS Exploit MIDAW?

One of the ways SyncSort for z/OS derives its superior performance is from the use of complex and highly sophisticated channel programs. SyncSort has used the data chaining I/O technique to read and write with non-contiguous storage areas for buffers to provide substantial CPU time benefits. When the MIDAW facility became available, SyncSort developers immediately took advantage of the improvements this facility had to offer. Modifications to SyncSort’s channel programs to incorporate MIDAWs result in additional performance savings of up to 10% in CPU time and 25% in elapsed time. SyncSort applications automatically use the MIDAW facility when possible, without any user intervention.

The savings achieved by SyncSort’s use of MIDAWs extend beyond individual sort applications. SyncSort’s use of MIDAW reduces the load of the channel subsystem. As with PAV exploitation, SyncSort’s use of MIDAWs is unique among sorting products.

Continue reading Part 3 in the series, this time covering zIIP.

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