ARCMDEXT: Space Management Exit

You can use the Start of change

space management exit End of change

(ARCMDEXT) to perform the following tasks:

Prevent selected data sets from Start of change

migration or class transition End of change

from level 0 volumes during volume migration processing, even when the data sets fulfill the selection criteria. For example, with this exit you could prevent migration of a large number of non-system-managed data sets instead of having to issue SETMIG command for each of the data sets.

For system-managed data sets whose management classes fulfill the migration selection criteria, you can use this exit to override the management class. Though this is not recommended, some users find that overriding the management class eliminates the introduction of many more management classes for these exception data sets.

Table 1 shows the rules governing migration versus class transition, based on eligibility and the return code from ARCMDEXT.

Table 1. Rules for migration versus class transition, based on eligibility and return code from ARCMDEXT
Eligibility Status	RC=0 (Allow Migration)	RC=8 (Disable Migration)	RC=52 (Disable Migration and Class Transition)
Eligible for migration only	Migration	Data set processing fails with RC=45, reason 92	Data set processing fails with RC=45, reason 92
Eligible for migration and class transition	Migration	Class transition	Data set processing fails with RC=45, reason 92
Eligible for class transition only	Class transition	Class transition	Data set processing fails with RC=45, reason 92

Override a data set condition called "in need of backup," which can prevent a data set from migrating to tape. A data set needs to be backed up if no current backup version exists and if the data set resides either on a non-system-managed volume or is a system-managed data set for which auto-backup is requested. Use caution, however, because if you use this exit to migrate the data set to tape, the data set will not be backed up by DFSMShsm.
Prevent compaction of a data set during volume migration if you have previously specified one of the following:
- SETSYS COMPACT(TAPEMIGRATE)
- SETSYS COMPACT(DASDMIGRATE)
- SETSYS COMPACT(ALL)
- SETSYS ZCOMPRESS(TAPEMIGRATE(YES))
- SETSYS ZCOMPRESS(DASDMIGRATE(YES))
- SETSYS ZCOMPRESS(ALL)
Override the target device type selection and route the data set to a tape migration level 2 volume. For system-managed data sets, it is easier to define a management class that migrates data sets from level 0 user volumes directly to level 2 migration volumes.
Control which data sets are eligible for reconnection when you specify either the SETSYS TAPEMIGRATION(RECONNECT(ALL)) command or the SETSYS TAPEMIGRATION(RECONNECT(ML2DIRECTEDONLY)) command. These data sets remain eligible for normal migration when the ARCMDEXT exit does not allow reconnection.
Indicate that the number of days that a data set is eligible for migration is passed in the input data structure. For system-managed data sets, eligibility age is based on the PRIMARY DAYS NON-USAGE management class attribute. For non-system-managed data sets, eligibility age is based on the number of days that are specified in the MIGRATE subparameter of either the ADDVOL PRIMARY command or the MIGRATE VOLUME command.
Note: Eligibility age fields may contain a negative value for both system-managed and non-system managed data sets that are processed for extent reduction and for system-managed data sets whose management class value is overridden by the DAYS(0) parameter of the MIGRATE VOLUME command. A negative value represents the number of days until the data set becomes eligible for normal migration.
Delay certain checkpointed data sets (for MVS or IMS™) from migrating from level 0 volumes until the need for restart has passed.
If a system-managed, physical-sequential data set is open during a checkpoint and is moved after that checkpoint, the program cannot restart from that checkpoint. This is true even for data sets that were only being read. A flag (DS1CPOIT) in the data set entry of the VTOC, indicates if a checkpoint was taken while the data set was open.

When an application program takes a checkpoint, the system records information about the status of that program in a checkpoint data set. This information includes the location on disk or tape where the application is currently reading or writing each open data set. If a data set that is open at the time of the checkpoint is moved to another location before the restart, you cannot restart the application from the checkpoint because the location-dependent information recorded by checkpoint/restart is no longer valid.

There are several system functions (for example, DFSMShsm) that might automatically move a data set without the owner specifically requesting it. To ensure that all checkpointed data sets remain available for restart, the checkpoint function sets the unmovable attribute for each SMS-managed sequential data set that is open during the checkpoint. An exception is the data set containing the actual recorded checkpoint information (the checkpoint data set), which does not require the unmovable attribute.

You can move checkpointed data sets when you no longer need them to perform a restart. DFSMShsm and DFSMSdss FORCECP(days) allow you to use operations such as migrate, copy, or defrag to move an SMS-managed sequential data set based on a number of days since the last access. DFSMShsm recall and DFSMSdss restore and copy are operations that turn off the unmovable attribute for the target data set.
If the DS1CPOIT bit is on (set to 1), the ARCMDEXT exit allows migration only if an installation-determined number of days have passed since the data set was last referred to. You can modify the field name, MGC_CKPOINT_DAYS, by issuing the following patch command:
```
    PATCH .MGCB.+70 X'03' VERIFY(.MGCB.+70 X'05')  /* change to 3 */
```
For more information about the checkpointed data set patch command, refer to z/OS DFSMShsm Implementation and Customization Guide.