Disk

In most cases, DFSORT's Blockset technique adjusts automatically to take advantage of the geometry of the particular IBM disk being used. This is especially true for work data sets, whose block sizes and distribution of data play crucial roles in the performance of DFSORT.

The location of input, output, and work data sets, as well as the speed of the disks on which they reside has a significant effect on the performance of a sort application. For best results, work data sets should be placed on different storage subsystems than the input and output data sets. This helps avoid channel, control unit path, and device contentions. To attain the maximum performance benefit, these data sets (or at least the input and output data sets) should be placed on the fastest disks so that DFSORT can take advantage of their speed.

In general, while DFSORT has no control over where its data sets are allocated, it does automatically optimize its access patterns based on data set location to achieve the best possible performance.

When allocating DFSORT work data sets on devices attached to non-synchronous storage control units or connected to ESCON channels, elapsed time may be degraded for certain applications. To avoid this degradation, it is especially important to follow the virtual storage guidelines described in Virtual storage guidelines and to ensure that DFSORT has an accurate knowledge of the size of the data set being sorted. See z/OS DFSORT Application Programming Guide for more details about non-synchronous storage subsystem considerations.

In addition to data set location, certain data set characteristics, such as block size, are also very important when considering performance. As mentioned before, DFSORT automatically chooses an optimal block size for its work data sets. Using DFSORT's installation option SDB=LARGE, SDB=INPUT or SDB=YES enables DFSORT to choose optimal block sizes for its output data sets on disk as well. Of less importance, using installation option SDBMSG=YES enables DFSORT to choose optimal block sizes for its message data sets on disk.