The ZS3-2 controller connects to external storage through a 4-port (4x4) SAS HBA. You can attach from one to 16 HDD-only or HDD/SSD Log cache disk shelves to the controller. You can also attach mixed disk shelf types behind the same controllers, but each chain must contain only the same disk shelf type. Directly connecting different disk shelf types is not supported. For more information, see Connecting to Attached Storage. See also Disk Shelf Hardware Overview for component specifications and diagrams.