Ideally, it is best to split volumes so that the SDMs receive an equal workload distribution, either by the disk subsystem or by the subsystem identifiers (SSIDs). In general, most customers split mirrored volumes among multiple SDMs at the disk subsystem level to simplify the allocation of volume pairs to an SDM. If you prefer a finer level of granularity, then splitting at the SSID level is also feasible. Assigning volumes to different SDMs on a logical storage subsystem (LSS) boundary should give you enough options to reasonably distribute activity over available SDMs.