Use this procedure to determine if you need to add more disks. Note that disks can be over-utilized by making a poor choice for the RAID profile and/or ZFS record size. In this case, it may be possible to reduce existing disk utilization by moving from RAIDZ to mirrored profiles and/or matching ZFS record sizes to client I/O sizes.
In addition, if the system is configured with no read- or write-optimized flash drives, then all of the I/O operations beyond DRAM are served by disks. For better performance, consider using flash for any workload that includes random reads or synchronous writes.
You may want to use more disks when at least 50% of existing disks are at least 70% utilized.