This is true...
Since the newer versions of OnTap (8.1+) changed the way multiple CPUs cores are utilized, its somewhat broken the normal methods of seeing CPU utilization like DFM and sysstat.
Our 6210 with 8 cores per head can show being pegged at near 100% cpu in DFM for most the day, however in reality its between 30-40% - we get a few annoying "cpu too busy" messages a day.
To get a more accurate reading of total CPU usage you have to use the "sysstat -m" command from the controller's CLI and look at the Avg column:
Filer> sysstat -m 1
ANY AVG CPU0 CPU1 CPU2 CPU3 CPU4 CPU5 CPU6 CPU7
97% 43% 45% 41% 40% 41% 44% 45% 44% 44%
97% 41% 44% 38% 38% 40% 42% 41% 43% 42%
96% 40% 42% 37% 38% 40% 42% 42% 42% 41%
100% 59% 57% 56% 55% 56% 61% 62% 60% 61%
99% 48% 50% 47% 44% 47% 51% 50% 50% 49%
99% 55% 55% 52% 53% 51% 57% 56% 57% 56%
96% 37% 38% 35% 36% 36% 39% 37% 39% 38%
"Any" is the metric DFM uses to show CPU usage - and as you can see above, its not really representative of actual filer load.
Furthermore - background processes will consume cpu, but will back-off in response to client workloads. so without looking at other metrics like throughput and latency, CPU% is rather irrelevant.
