Using on-demand migration

On-demand migration is a space management function that performs near immediate space management on eligible SMS-managed volumes that exceed the specified volume high threshold, instead of waiting for interval migration to run at the top of each hour. On-demand migration migrates data sets to ML1 or ML2 volumes and transitions data sets from one class of user-accessible volumes to another until the low threshold is reached or there are no eligible data sets remaining.

During on-demand migration:

DFSMShsm listens for an ENF72 event code signal from SMS indicating a volume has exceeded the high threshold and queues the volume for processing.
An eligible volume is selected from the queue for space management.
The percent of allocated space on the selected volume is checked.
A message is issued for unmatched data sets.
Temporary data sets are deleted.
Data sets that are eligible for class transitions are transitioned. If a data set is eligible for both a class transition and migration, the data set will only be migrated. This prevents the data set from being moved for a transition, only to be later migrated.
Expired data sets are deleted.
Data sets that are eligible for migration are migrated.

The following functions that are performed during primary space management are not performed during on-demand migration:

Partial release.
Extent reduction.

Specific on-demand migration eligibility requirements must be satisfied both at the volume level and the data set level. See On-demand migration eligibility requirements.

Enabling on-demand migration

There are two methods of enabling on-demand migration:

Add the SETSYS ONDEMANDMIGRATION(Y) command to the ARCCMDxx parmlib member of SYS1.PARMLIB. Then restart DFSMShsm.
Issue the SETSYS ONDEMANDMIGRATION(Y) command after DFSMShsm is started.

On-demand migration is disabled by default.

To determine if on-demand migration is enabled or disabled, issue QUERY SETSYS. Message ARC0139I is issued with the query response.

To use on-demand migration after it is enabled, eligibility requirements must be satisfied. See On-demand migration eligibility requirements.

Disabling on-demand migration

There are two methods of disabling on-demand migration:

Add the SETSYS ONDEMANDMIGRATION(N) command to the ARCCMDxx parmlib member of SYS1.PARMLIB. Then restart DFSMShsm.
Issue the SETSYS ONDEMANDMIGRATION(N) command after DFSMShsm is started.

On-demand migration eligibility requirements

There are two sets of requirements that must be satisfied in order for a volume to be eligible for on-demand migration. The first set of requirements must be satisfied at the volume level. The second set of requirements must be satisfied at the data set level.

Requirements that must be satisfied at the volume level for on-demand migration to be performed are:

The volume is an SMS-managed volume.
The volume belongs to a storage group defined with AM=Y (Auto Migrate, yes).
The volume exceeds the high threshold.
The SETSYS ONDEMANDMIGRATION(Y) setting is in effect.

Requirements that must be satisfied at the data set level for a data set to be migrated or transitioned from an eligible volume are:

The management class for the data set is configured to allow both command or auto migration (COMMAND OR AUTO MIGRATE=BOTH)
For transitions:
- The age of the data set has exceeded the management class TIME SINCE CREATION attribute value.
- The last referenced date has met or exceeded the management class TIME SINCE LAST USE attribute value.
- The current date has met or exceeded the PERIODIC time period specified in the management class.
For migrations, the inactive age of the data set has exceeded the management class PRIMARY DAYS NON-USAGE attribute value.
If the data set is a candidate to be backed up:
- It is not eligible for migration if the migration of the data set is directed to tape.
- It is not eligible for a transition if the management class TRANSITION COPY TECHNIQUE specifies a technique other than STANDARD.

On-demand migration is not performed on non-SMS-managed volumes or SMS-managed volumes that belong to a storage group defined with AM=I (Auto Migrate, interval). Interval migration is performed instead.

On-demand migration in a multiple DFSMShsm host environment

In a multiple DFSMShsm host environment:

Each host configured to perform on-demand migration selects eligible volumes for on-demand migration up to the configured maximum number of migration tasks (that is, the SETSYS MAXINTERVALTASKS command). As volumes are processed, each host selects from the remaining volumes until all volumes have been processed.

Note: MAXEVENTDRIVENMIGTASKS or the MAXEDMT alias can be used instead of the MAXINTERVALTASKS parameter.
You should distribute on-demand migration workload evenly across all hosts configured to perform on-demand migration.
You should ensure there are enough hosts to perform other critical DFSMShsm tasks.

On-demand migration usage notes and other considerations

Neither interval migration or on-demand migration is performed on SMS-managed volumes that belong to a storage group defined with AM=P (Auto Migrate, primary) or AM=N (Auto Migrate, no).
On-demand migration is paused when interval migration or primary space management are started on the same DFSMShsm host performing on-demand migration. On-demand migration will start again after interval migration or primary space management has ended, if there are eligible volumes to be processed.
Notification of the number of volumes concurrently queued for on-demand migration is issued to the operator's console (highlighted message ARC1901E). The default notification limit is 100 volumes. For more information about configuring the notification limit, see Specifying an on-demand migration number of concurrently queued volumes notification limit
After a volume is processed using on-demand migration it can remain at or above the high threshold. This happens when there are not enough eligible data sets on the volume to be space managed. To prevent continuously processing a volume that remains above the high threshold, an on-demand migration timer is started. This timer prevents on-demand migration from running again for 24 hours (default) on all volumes that remained at or above the high threshold during its duration. When the timer expires, on-demand migration is performed again on all of the volumes that remain at or above the high threshold. A patch is provided to change the frequency at which volumes at or above the high threshold are selected for on-demand migration again. See z/OS DFSMShsm Implementation and Customization Guide.
When on-demand migration is enabled, the SETSYS INTERVALMIGRATION setting and the SETSYS NOINTERVALMIGRATION setting are ignored for on-demand migration eligible volumes.
If you want to perform on-demand migration on TMM storage groups, you must convert AM=I (Auto Migrate, interval) TMM storage groups to AM=Y.
If you want the operator to be notified of the space allocation of each volume during the on-demand migration space check, issue the SETSYS MONITOR(SPACE) command.
Data sets on SMS-managed volumes are processed in the following order:
1. Data sets eligible for a class transition and migration will be migrated.
2. Data sets eligible for a class transition will be transitioned.
3. Data sets eligible for migration only will be migrated. For this phase, data set size is a determining factor for the order and priority in which data sets are migrated. The largest eligible data set migrates first, followed by the next largest. The priority for migrating data sets from volumes that belong to storage groups defined with AM=I applies when the low threshold exceeds 5% only.
In order for a data set to be migrated or transitioned, the date of the current backup copy should be greater than the date-last-referenced by at least one day.