According to support this is normal. "The only reason you would have 0 across all these fields is if the filer was turned off."
Even so, it is strange that sometimes it's really low (close to 0) and sometimes really high (I believe I saw values around 1GB/s)........
CPU Total Net kB/s Disk kB/s Tape kB/s Cache Cache CP CP Disk
ops/s in out read write read write age hit time ty util
25% 0 0 0 125648 16 0 0 20s 93% 0% - 80%
23% 0 1 0 186792 0 0 0 20s 95% 0% - 94%
22% 0 1 0 193341 21 0 0 20s 94% 0% - 81%
88% 0 1 0 801715 0 0 0 36s 84% 66% Z 91%
48% 0 1 0 466656 1859 0 0 36s 100% 56% : 51%
1% 0 1 0 7168 5 0 0 36s 100% 0% - 5%
52% 0 1 0 989968 16 0 0 36s 90% 0% - 93%
48% 0 1 0 935253 0 0 0 36s 100% 0% - 98%
47% 0 1 0 930139 16 0 0 36s 100% 0% - 95%
51% 0 0 0 993280 5 0 0 36s 100% 0% - 97%
50% 0 1 0 984752 16 0 0 36s 100% 0% - 97%
14% 0 1 0 263008 1485 0 0 36s 99% 16% T 33%
10% 0 1 0 193541 21 0 0 36s 100% 0% - 24%