How the system satisfies the secondary space request

For many data sets, the primary quantity does not need to be big enough for the entire data set. Code a secondary quantity to be used only if the data set exceeds its originally allocated space. The system tries to obtain the secondary space contiguous to the last extent of space allocated to the data set. But, if you specify the secondary quantity in cylinders, in blocks, or in records with the ROUND subparameter, then the secondary space allocated to the data set starts at the beginning of a cylinder.

Note: BDAM data sets cannot be extended.

Volume for secondary space for NEW or MOD data set

For data sets whose disposition is NEW or MOD, the system allocates this space on the same volume as the primary quantity until one of the following occurs:

The volume does not have enough space available for the secondary quantity.
16 extents, less the number of extents for the primary quantity and user label space, have been allocated to the partitioned data set (PDS).
123 extents, less the number of extents for the primary quantity and user label space, have been allocated to the partitioned data set extended (PDSE).

Then, the system allocates the secondary quantity on another volume only if the DD statement requested more than one volume in the VOLUME parameter or, for a specific volume request, requested more volumes than devices (which implies unit affinity).

If the DD statement makes a nonspecific volume request and the system could possibly allocate a permanently resident volume, code PRIVATE in the VOLUME parameter.

Volume for secondary space for OLD data set

When allocating a secondary quantity for a data set whose status is OLD, that is, an existing data set being written over or a preallocated data set, the system checks for a next volume. If a next volume exists, the system looks for a secondary quantity already allocated in it. If the system finds a secondary quantity, the system uses that space. If the system finds no space already allocated, the system allocates the secondary quantity on that next volume. If a next volume does not exist, the system allocates the secondary space on the current volume.

Secondary request only for current execution

A secondary quantity can be requested when creating a data set or when retrieving an existing data set, whether or not you coded a secondary quantity in the original request. A secondary request for an existing data set is in effect only for the duration of the job step and overrides an original request, if one was made.

Secondary requests in blocks

If you request space in terms of average block length, the system allocates the least number of tracks required to contain the request.