When you code DISP=OLD or DISP=SHR, space is allocated to system-managed multivolume (non-extended-format) data sets with the guaranteed space attribute in the storage class, as follows:

1. Initially, the system preallocated primary space on all the volumes when you coded DISP=NEW.
2. When the allocated space on each volume is used up, the system switches to the next volume. Some volumes might already have secondary space allocations because you extended the data set when you coded DISP=NEW or DISP=MOD earlier. The system will use those secondary allocations.
3. The existing space on the next volume is then used.
4. The system will attempt to allocate new space only on the last volume. On that volume secondary amounts continue to be allocated until the volume is out of space or the data set extent limit is reached.

The system works this way so that it is similar to nonguaranteed preallocated space on non-SMS volumes.