If the VTPOOL parmlib member is not currently being used to define Virtual Volumes ranges, available scratch VOLSERs are maintained in a single Scratch Pool. The Scratch Synchronization Job makes two passes through the Virtual VOLSERs in the Global VCAT.
The first pass reads the Virtual Volume Entries in the Global VCAT and checks their matching entry in the Scratch Pool. If an entry is not active, it is updated to make it active which makes the VOLSER available for reuse. As entries are updated, the pointer to the next available scratch VOLSER is automatically updated.
The automatic pointer update causes the Virtual VOLSERs to always be reused in order from first to last. As scratch VOLSERs are used throughout the day, the pointer moves down through the pool. When the Scratch Synchronization Job runs, if it makes a previous entry available in the pool, the pointer is reset back to that entry. This entry will be the entry used to satisfy the next scratch mount request.
In the following example, V00006 is currently the next available scratch VOLSER. If the Scratch Synchronization Job runs and makes V00001, V00002, and V00004 available for reuse, the pointer will be reset to V00001. The next scratch mount request will reuse V00001 and turn it off. V00002 will then be the next available VOLSER.
BEFORE SYNCH AFTER SYNCH AFTER NEXT MOUNT V00000 V00000 V00000 V00001 V00001 On <- V00001 V00002 V00002 On V00002 On <- V00003 V00003 V00003 V00004 V00004 On V00004 On V00005 V00005 V00005 V00006 On <- V00006 On V00006 On
Due to the pointer reset, the first entries in the Scratch Pool will always be reused before the last entries are ever used. If the Virtual VOLSER range is V00000-V99999, V00000 will be reused many times and V99999 might never be used.
The second pass uses a list of all Virtual Volumes in scratch status in the Tape Management System to update only the Virtual Volume Entries in the Global VCAT that are in active status. These newly scratched Virtual Volumes are not activated in the Scratch Pool as that is the function of the first pass and it has already been executed. These Virtual Volumes will be activated in the Scratch Pool by the next execution of the Scratch Synchronization Job. If the job is being run once every 24 hours, you have that amount of time to recover accidentally scratched Virtual Volumes before they eligible for reuse.
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