Reconnecting data sets using fast subsequent migration

Using the standard migration and recall method, a data set can be migrated to an ML2 tape volume and later be recalled. When the data set is recalled, the migrated copy of the data set remains on the ML2 tape volume and is marked not valid in the CDS record for the data set. If the recalled data set is later migrated using the standard migration method, it is moved to a new ML2 tape volume (even if the data set has not changed). This process results in unnecessary data movement and causes the original ML2 tape volume to become fragmented which requires the tape to be recycled.

Fast subsequent migration allows an unchanged data set that is recalled from a single ML2 tape volume to be reconnected to that ML2 tape volume during data set or volume migration. Using fast subsequent migration:

eliminates the unnecessary data movement and tape mounts caused by migrating a data set again that is unchanged.
involves creating and updating DFSMShsm control records only.
reduces the need for recycling the ML2 tape.

Specific fast subsequent migration eligibility criteria must be satisfied both at time of recall and when a fast subsequent migration reconnect is attempted on a data set. See Fast subsequent migration (reconnection) eligibility requirements.

Enabling fast subsequent migration

There are two methods of enabling fast subsequent migration:

Add the SETSYS TAPEMIGRATION command with the RECONNECT(ALL) or RECONNECT(ML2DIRECTEDONLY) parameter to the ARCCMDxx parmlib member of SYS1.PARMLIB. Then restart DFSMShsm.
Issue the SETSYS TAPEMIGRATION command with the RECONNECT(ALL) or RECONNECT(ML2DIRECTEDONLY) parameter after DFSMShsm is started.

To use fast subsequent migration after it is enabled, the data set must also satisfy fast subsequent migration eligibility requirements. See Fast subsequent migration (reconnection) eligibility requirements.

Disabling fast subsequent migration

There are two methods of disabling fast subsequent migration:

Add the SETSYS TAPEMIGRATION command with the RECONNECT(NONE) parameter to the ARCCMDxx parmlib member of SYS1.PARMLIB. Then restart DFSMShsm.
Issue the SETSYS TAPEMIGRATION command with the RECONNECT(NONE) parameter after DFSMShsm is started.

Fast subsequent migration (reconnection) eligibility requirements

There are two sets of requirements that must be satisfied in order for a data set to be eligible for fast subsequent migration. The first set of requirements must be satisfied when a data set is recalled. The second set of requirements must be satisfied when a fast subsequent migration reconnect is attempted on a data set. In addition to the two sets of eligibility requirements, there are conditions which always prevent fast subsequent migration regardless of whether all other requirements are satisfied.

Conditions that always prevent fast subsequent migration are:

The data set is an HFS file.
The data set is a VSAM data set with an alternative index (AIX) or path defined.
The data set has changed since it was recalled.
The data set spans multiple ML2 tape volumes.
The MIGRATE command was issued with the CONVERT parameter.
The ARCHMIG macro is used with the FORCML1=YES parameter to direct the data set to an ML1 volume.
The SETSYS TAPEMIGRATION command with the RECONNECT(NONE) setting is in effect.

Requirements the must be satisfied when the data set is recalled are:

The data set is recalled from a single ML2 tape volume.
The SETSYS USERDATASETSERIALIZATION setting is in effect.
The SETSYS TAPEMIGRATION command with the RECONNECT(ALL) or RECONNECT(ML2DIRECTEDONLY) setting is in effect.

Requirements that must be satisfied when a fast subsequent migration reconnect is attempted on the data set are:

The data set was marked reconnectable when recalled.
No changes have been made to the data set since it was recalled.
The catalog entry for the data set has not been altered since it was recalled.
The MCDS record for the data set has not been deleted (for example, when migration cleanup is performed).
The ML2 tape volume has not been recycled since the data set was recalled.
The SETSYS USERDATASETSERIALIZATION setting is in effect.
The SETSYS TAPEMIGRATION command with the RECONNECT(ALL) or RECONNECT(ML2DIRECTEDONLY) setting is in effect.

Note:

Data sets that were migrated to an ML2 tape volume without requiring a backup copy (such as when using tape mount management) are eligible for fast subsequent migration as long as all other requirements are satisfied.
If a data set fails fast subsequent migration it is still eligible for standard migration.
A default date for fast subsequent migration is predicted when an eligible data set is recalled. The default date is the date the eligible data set was recalled plus the number of days the data set was unreferenced when it was last migrated. Fast subsequent migration is performed if the eligible data set is not referenced before that date and if the data set satisfies all fast subsequent migration and standard migration requirements.

Delaying migration cleanup of fast subsequent migration eligible data sets

Migration cleanup is performed on the MCDS records of fast subsequent migration eligible data sets a default number of days after the predicted remigration date if the data set was not reconnected (that is, fast subsequent migration was not performed). The default number of days is the default value of the SETSYS MIGRATIONCLEANUPDAYS command reconnectdays parameter.

Specify a reconnectdays value greater than the default to improve the likelihood of performing fast subsequent migration on eligible data sets by delaying migration cleanup of the MCDS records for those data sets.

Attention: The size of the MCDS might need to be increased to accommodate the extended life of MCDS records for fast subsequent migration eligible data sets.

Programming interface information

Using ARCMDEXT to control fast subsequent migration eligibility

You can use the ARCMDEXT installation exit to redirect migrations that target ML1 volumes so that they target ML2 tape volumes instead. If you specified the SETSYS TAPEMIGRATION command with the RECONNECT(ALL) parameter, you can use this exit to deny reconnection for specific data sets. The ARCMDEXT exit is passed information in the parameter list, when fast subsequent migration is performed, as to whether a specific data set is reconnectable.

End of programming interface information