Answering the concern of adding power consumption (including heat), additional rack space and etc. for additional enclosures, as well as performance concerns is the larger enterprise storage chain. 3.5″ SATA or SAS drives are used for mass storage (including potentially disk based backup instead of tape), 2.5″ and/or 3.5″ SAS or SSDs are used as the next tier of storage. With these large systems, disks are often contained in multiple shelves requiring a Fibre Channel, SAS, or Fibre Channel over IP connection between the dedicated controller (or controllers) and the disks.
To answer the need for more performance, many vendors such as HP, NetApp (PAM II module pictured above), EMC, and etc are using local NAND cache, often using a PCIe add-in card. This gives the system very high performance solid state caching inside the controller enclosure instead of through external cabling. Things like having 4TB of NetApp’s PAM II PCIe modules are probably out of reach for most small businesses and home server aficionados, however they do serve a great value in enterprise applications. As an aside, this is similar to what happens when one uses a RAID controller that can incorporate SSD caching on the hardware RAID card.