Figure 1 shows how the first contiguous group of unallocated tracks on the target volume that is large enough to contain the source spool data set is used for the migration. The highest track that is used on the source spool data set determines the size of the data set to be migrated, as shown in Figure 1. JES2 adds additional space to that size to allow for internal structures that are used to track the progress of migrating data. These structures are stored on the target volume, so their total size is added to the source data set size to determine the total amount of free space that is required on the target volume.