Related Concepts

You should understand the process the system uses to select the tape devices to be allocated. The following steps describe how the system processes the tape requests for each job step:

The system initializes fields in the tape allocation subsystem interface mapping (IEFSSTA, called SSTA in this section). The SSTA mapping consists of:
- An SSTA header (one for each jobstep) that contains general information about the jobstep
- A DD section (one for each DD statement or dynamic allocation request requiring a non-SMS managed tape device) that contains information about the DD
- A device request section (one for each device indicated on the DD statement) that contains information about the tape device request
- An eligible device array entry (one for each eligible device) that contains selection criteria.
In initializing the eligible device array entry, the system considers the following facts about the tape device requests and the characteristics of available devices:
- The type of requests (such as a request for a private, scratch, or specific volume)
- Unit information on the requests
  The system uses the eligible device table (EDT) to determine which devices are eligible to satisfy the request.
- Characteristics of each eligible tape device, such as:
  - Does the device already have the requested volume mounted
  - Is the device online or offline
  - Is the device dedicated or automatically switchable.
  These characteristics are reflected in bits in the SSTAIBMM field.
  
  Several of the IBM® eligibility bits are set based on whether a volume is already mounted on the device. The following helps you understand the conditions that can cause a volume to be already mounted on a device.
  A volume may already be mounted for any one of the following conditions:
  - A volume was premounted as the result of a MOUNT command issued by the operator
  - A volume was inserted into the drive by the operator, but no MOUNT command was issued by the operator
  - A volume is mounted on a drive because a prior step in the same job passed a data set to a subsequent step or the request specified RETAIN
  - A volume is mounted on a drive because it is in use by another job
  Within an eligible device array entry, the order of the characteristics reflects their relative importance. For example, whether a specific device is mounted is more important than whether the device is automatically switchable.
  
  The system then builds a list of eligible tape devices and associated eligibility values generated from bits in the SSTAIBMM field in the eligible device array entry.
  
  At this point, the system issues SSI function code 78, passes the SSTA (including the eligible device array), and gives your Tape Device Selection function routine a chance to affect the selection. When the function routine gets control, it can set bits in the SSTAUSRM field. If SSTAUSRM bits are set, the system generates eligibility values that combine SSTAUSRM settings and SSTAIBMM settings.
Based on the list of eligible devices and associated eligibility values built in step 1, the system selects the optimal device to allocate for the request.
When the UNIT parameter of the request specifies an esoteric that consists of devices from multiple generics (for example, 3490 and 3590-1), the device preference order is used in conjunction with other IBM and user settings to choose the optimal device. Devices belonging to the generic that is higher in preference order naturally have preference over those belonging to a generic lower in the preference order. See z/OS HCD Planning for an explanation and directions for specifying the device preference order.

You cannot specify the generic device type and it cannot be overridden in either IBM or user settings of SSI78. You can apply user settings to influence choice of an optimal device within a generic of a multi generic esoteric. To influence which generic contains the optimal device either change the device preference order using HCD or use the SSTAINEL setting for all devices in the generic which are not desirable. You can determine device type using EDTINFO (for details, see the SSTADNUM input parameter SSTADNUM).

Table 1 shows the logical relationship between the system settings and the user settings in the eligible device array entry. The first column shows the 1-bit fields the system sets in SSTAIBMM; the second column shows the 1-bit fields the function routine can set in SSTAUSRM. The criteria are listed in order of importance, from top to bottom. For example, the most important criteria are:

SSTAINEL, a user field that can remove the device from consideration
SSTADMND, a system field that identifies the device as the one specified on the DD statement.

The table shows how the user criteria interleave with system criteria.

Table 1. Relationship between System and User Criteria
Importance	System criteria (SSTAIBMM)	User criteria (SSTAUSRM)
1		SSTAINEL
2	SSTADMND
3		SSTAUS01
4		SSTAUS02
5	SSTAONUN
6		SSTAUS03
7		SSTAUS04
8	SSTANAFH
9		SSTAUS05
10		SSTAUS06
11	SSTASPCM
12		SSTAUS07
13		SSTAUS08
14	Generic device type not specified by a bit
15		SSTAUS09
16		SSTAUS10
17	SSTAACL1
18		SSTAUS11
19		SSTAUS12
20	SSTAACL2
21		SSTAUS13
22		SSTAUS14
23	SSTAACL3
24		SSTAUS15
25		SSTAUS16
26	SSTAVOLM
27		SSTAUS17
28		SSTAUS18
29	SSTANVOL
30		SSTAUS19
31		SSTAUS20
32	SSTAWVOL
33		SSTAUS21
34		SSTAUS22
35	SSTAAVOL
36		SSTAUS23
37		SSTAUS24
38	SSTAANAS
39		SSTAUS25
40		SSTAUS26

Descriptions of SSTAIBMM fields are found in Input Parameters; descriptions of SSTAUSRM fields are found in Output Parameters.