If the system disk pool fills to capacity, the system will end normal activities. If this occurs, you must perform an IPL of the system, and take corrective action (such as deleting objects) to prevent this from recurring.
You can also specify a threshold that, when reached, warns the system operator of a potential shortage of space. For example, if you set the threshold value at 80 for the system disk pool, the system operator message queue (QSYSOPR) is notified when the system disk pool is 80% full. A message is sent every hour until the threshold value is changed, or until objects are deleted or transferred out of the system disk pool, or until disk units are added to the system disk pool. If you ignore this message, the system disk pool fills to capacity, and the system ends abnormally.
You can use a third method of preventing the system disk pool from filling
to capacity by using the QSTGLOWLMT and QSTGLOWACN system values. For more
information, refer to "How to Change the Storage Threshold for the System
Auxiliary Storage Pool" in Backup
and Recovery book 
.