For optimal performance, keep in mind the following:
Rule 1 -- All "data" disks contained within a head node or JBOD must have the same rotational speed (media rotation rate). The ZFSSA software will detect misconfigurations and generate a fault for the condition.
Recommendation 1 -- Due to unpredictable performance issues, avoid mixing different disk rotational speeds within the same pool.
Recommendation 2 -- For optimal performance, do not combine JBODs with different disk rotational speeds on the same SAS fabric (HBA connection). Such a mixture operates correctly, but likely results in slower performance of the faster devices.
Recommendation 3 -- When configuring storage pools that contain data disks of different capacities, ZFS will in some cases use the size of the smallest capacity disk for some or all of the disks within the storage pool, thereby reducing the overall expected capacity. The sizes used will depend on the storage profile, layout, and combination of devices. Avoid mixing different disk capacities within the same pool.