On several occasions a number of our Red Hat (6 & 7) servers have stopped reporting Disk Utilization .1.3.6.1.2.1.25.2.3.1.6.  This usually happens after a reboot or in the last instance when the OpManager service shutdown unexpectedly.  When the systems are checked in OpManger all of the other SNMP counters, queries are working properly.  I've run a rediscovery on the affected servers with no success, the only way to get Disk Utilization working properly is to delete the system completely and then add it back in.  This is a time consuming process and we lose history not to mention reconfiguring monitors.  As a note this happens to both physical and VM servers