UNIT Parameter

Purpose

The UNIT parameter specifies the device type on which a data set should be allocated. It identifies the general type of device, a specific device, or a group of devices defined by the installation. The UNIT parameter is crucial for controlling where data sets are stored in the mainframe environment.

Syntax

Basic Format

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//ddname DD UNIT=unit-information

Extended Format

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//ddname DD UNIT=(unit-type[,unit-count][,DEFER])

Parameter Values

Unit Type

Unit Count

Specifies the maximum number of devices to be allocated to the data set. Valid values are 1-59. This is typically used for multi-volume data sets.

DEFER Option

Requests deferred mounting of volumes. The system allocates the device but does not mount the volume until the data set is opened.

Common Device Types and Groups

Examples

Basic UNIT Specification

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//OUTDD DD DSN=USER.DATA.SET,DISP=(NEW,CATLG,DELETE),
//         UNIT=SYSDA,SPACE=(CYL,(10,5))

Allocates a new data set on any device in the SYSDA group

Specific Device Type

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//DISKDD DD DSN=PROD.MASTER.FILE,DISP=SHR,
//         UNIT=3390

Requests a 3390 disk device for an existing data set

Tape Data Set

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//TAPEDD DD DSN=BACKUP.WEEKLY,DISP=(NEW,KEEP),
//         UNIT=TAPE,LABEL=(1,SL)

Creates a new data set on a tape device

Multi-Volume Data Set

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//BIGDD DD DSN=LARGE.DATA.SET,DISP=(NEW,CATLG,DELETE),
//        UNIT=(SYSDA,3),SPACE=(CYL,(500,50)),
//        VOL=SER=(VOL001,VOL002,VOL003)

Allocates a data set that can span up to 3 SYSDA devices, with specific volume serial numbers

Deferred Mounting

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//ARCHDD DD DSN=ARCHIVE.MONTHLY,DISP=OLD,
//         UNIT=(TAPE,,DEFER)

Requests a tape device but defers mounting until the data set is opened

Temporary Data Set

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//TEMPDD DD DSN=&&TEMP,DISP=(NEW,DELETE),
//         UNIT=VIO,SPACE=(TRK,(5,2))

Creates a temporary data set using virtual I/O for high performance

Specific Device Address

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//SPECDD DD DSN=CRITICAL.DATA,DISP=OLD,
//         UNIT=150

Requests a specific device by its address (150)

UNIT Selection Process

1. SMS Management: For SMS-managed data sets, UNIT may be ignored as SMS controls placement
2. Existing Data Sets: For OLD, SHR, or MOD of existing data sets, UNIT is used if the system needs to allocate the device
3. New Data Sets: For NEW or MOD of new data sets, UNIT determines eligible devices for allocation
4. Generic vs. Specific: Generic names (like SYSDA) give the system more flexibility in allocation
5. Unit Count: With multiple units specified, the system allocates devices for each volume up to the unit count
6. Affinity: The system tries to minimize the number of devices used while honoring the requirements

Use Cases

Limitations and Considerations

• SMS Override: For SMS-managed data sets, the UNIT parameter may be ignored as SMS controls placement
• Installation Dependencies: Available unit names and groups are installation-specific and may vary
• Device Availability: Allocation fails if the requested device type is not available
• ACS Routines: SMS Automatic Class Selection routines may influence device selection regardless of UNIT
• UNIT vs. STORCLAS: For SMS-managed data sets, STORCLAS typically has precedence over UNIT
• Hardware Dependencies: Specific device types may have hardware-specific requirements
• Unit Count Restrictions: The maximum unit count is 59, but practical limits are usually much lower

Common Mistakes

• Specifying Unavailable Devices: Requesting device types that don't exist at the installation
  jcl

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  //ERROR1 DD DSN=TEST.DATA,DISP=NEW,
  //         UNIT=3380  // If 3380 devices no longer exist

• Ignoring SMS Management: Specifying UNIT for data sets that will be SMS-managed anyway
  jcl

  1
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  //ERROR2 DD DSN=SMS.MANAGED.DATASET,DISP=(NEW,CATLG),
  //         UNIT=SYSDA  // May be ignored by SMS

• Unit Count Mismatch: Specifying unit count that doesn't match volume count
  jcl

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  //ERROR3 DD DSN=MULTI.VOLUME,DISP=(NEW,CATLG),
  //         UNIT=(SYSDA,2),VOL=SER=(VOL1,VOL2,VOL3)
  //         // Only 2 units for 3 volumes

• Forgetting DEFER for Tapes: Not using DEFER for tapes that aren't immediately needed
  jcl

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  //ERROR4 DD DSN=SELDOM.USED.TAPE,DISP=SHR,
  //         UNIT=TAPE  // Should use DEFER if not used immediately

• Using Specific Devices Unnecessarily: Specifying a particular device when any would do
  jcl

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  //ERROR5 DD DSN=GENERAL.DATA,DISP=(NEW,CATLG),
  //         UNIT=3390-2  // Too specific, SYSDA would be better

• Omitting UNIT for New Data Sets: Not specifying UNIT for new non-SMS data sets
  jcl

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  //ERROR6 DD DSN=NEW.DATASET,DISP=(NEW,CATLG),
  //         SPACE=(CYL,(10,5))  // Missing UNIT parameter

Best Practices

• Use Generic Names: Prefer generic unit names (SYSDA, TAPE) over specific device types when possible
• Follow Installation Standards: Use standard unit names defined by your installation
• Consider SMS Management: Remember that SMS may override UNIT specifications
• Use DEFER for Tapes: Specify DEFER for tape data sets that aren't used immediately
• Match Unit Count to Volumes: Ensure unit count is appropriate for the number of volumes
• Use VIO for Temporary Data: Consider using VIO for small, temporary data sets
• Balance Performance: Distribute high-activity data sets across different devices
• Document Special Requirements: If using specific device types, document the reason
• Be Consistent: Use consistent unit specifications across related jobs
• Check Availability: Ensure requested device types are available at all execution sites

Related Concepts

• VOLUME Parameter - Often used with UNIT to specify volume serial numbers
• DD Statement - The UNIT parameter is specified on the DD statement
• SPACE Parameter - Used with UNIT for allocating space on direct access devices
• STORCLAS Parameter - SMS storage class that may override UNIT specifications
• MGMTCLAS Parameter - SMS management class that affects data set handling

Related Pages

• DD Statement
• DISP Parameter
• SPACE Parameter
• VOLUME Parameter
• DSN Parameter
• LABEL Parameter