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 David, 

You are correct there are only 16 extents total allowed no matter what the combination of the primary or secondary is allocated. In my experience of 30 years we always tried for the primary allocation to be the majority or all of the DSN size (quicker to read back in) and structured our data center in that way however, there were times when it couldn't be done therefore the secondary allocation would be utilized. If the contiguous primary allocation couldn't be found after the system checked all the pack pool then the job abended and the proper steps would be taken to either allocate a lower primary allocation of what continguous space was available(to get the job up and running), or add more packs to the pool. The secondary allocation was for growth of the DSN without having the job abend. Your point well taken. Thanks for the reminder!!

Janet (from linkedin)