Using FreeNAS IOstat

zpool iostat are very low level tools that offer concrete information about the throughput, latency, and some of the usage patterns of individual disks

root@freenas[~]# zpool iostat
                 capacity     operations    bandwidth
pool          alloc   free   read  write   read  write
------------  -----  -----  -----  -----  -----  -----
Pool          5.68T  3.38T     50    778  3.67M  12.9M
freenas-boot  1.01G  38.5G      0      0  1.96K    346
------------  -----  -----  -----  -----  -----  -----
more ...