You can combine SAS-connected storage shelves containing performance disks and SAS-connected storage shelves containing capacity disks within the same stack, although this configuration is not recommended.
Each SAS-connected storage shelf can contain only one class of disk (capacity, performance, or SSDs). The only exception to this rule is if the shelf is being used for a Flash Pool aggregate. In that case, for some SSD sizes and shelf models, you can combine SSDs and HDDs in the same shelf. For more information, see the Hardware Universe.