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
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Pool 5.68T 3.38T 50 778 3.67M 12.9M
freenas-boot 1.01G 38.5G 0 0 1.96K 346
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