DASD device allocation

Device allocation selects the most responsive DASD devices as candidates for permanent data sets on mountable devices (JCL specifies nonspecific VOLUME information or a specific volume and the volume is not mounted).

The ability of SRM to control DASD device allocation is limited by the decision an installation makes at system installation time and at initial program loading (IPL) time, as well as by the user's JCL parameters. SRM can only apply its selection rules to a set of DASD devices that are equally acceptable for scheduler allocation. This set of devices does not necessarily include all the DASD devices placed in an esoteric group during system installation. At that time, an esoteric group is defined by the UNITNAME macro and entered in the eligible device table (EDT). During system installation each esoteric group is partitioned into subgroups if either of the following conditions occurs:

The group includes DASD devices that are common with another esoteric group.
The group includes DASD devices that have certain generic differences. System installation partitions only esoteric groups that consist of magnetic tape or direct access devices.

For example, assume that you specify the following at system installation time:

UNITNAME=DASD,UNIT=((470,7),(478,8),(580,6))
UNITNAME=SYSDA,UNIT=((580,6))

Because of the intersection with SYSDA (580,6), the DASD group is divided into two subgroups: (470,7) and (478,8) in one subgroup and (580,6) in the other.

Allocation allows SRM to select from only one subgroup at a time. After allocating all devices in the first subgroup, allocation selects DASD devices from the next subgroup. Using the previous example, when a job requests UNIT=DASD, allocation tells SRM to select a device from the first group (470-476 and 478-47F) regardless of the relative use of channel paths 4 and 5. After all of the DASD devices in the first group have been allocated, allocation tells SRM to select devices from the second group (580-585).