Fundamental Concepts in Storage Interconnects

Just because a Fibre Channel network connects systems to storage, there is no reason why Fibre Channel has to be used inside a SAN storage subsystem. There are many ways to mix external and internal networks and buses, as we will explore in this chapter. Obviously, a NAS or iSCSI subsystem that connects to an Ethernet network needs to use something other than Ethernet as an interconnect, because Ethernet is not used (yet) on storage devices.

From a technical analysis, the interconnect technology for a storage device is independent of most of the other components inside a device. For example, the internal storage controllers and caching technology of a SCSI drive are likely to be specifically developed for SCSI operations. The platters, heads, disk arms, actuators, motors, and power supplies could also be used in any ATA, Fibre Channel, SATA, USB, or FireWire disk drive.

Storage devices are classified by their interconnect. For instance, disk drives are sold and marketed as SCSI, ATA, Fibre Channel, USB, FireWire, or other types of drives. Device prices also depend somewhat on the class of device. Fibre Channel and SCSI devices are typically more expensive than their ATA counterparts. The relative cost of an interconnect used for a particular device is commensurate with the cost and quality of other components used in a device. For instance, the cost of a SCSI or Fibre Channel disk drive is higher than the corresponding cost of a typical ATA disk drive.

Historically, innovations in disk drives have been introduced first in SCSI and Fibre Channel disks and then later implemented in ATA drives as the technology becomes cheaper to produce. However, as SATA drives become more commonly used in enterprise storage subsystems, they will likely see innovations as soon as SCSI and Fibre Channel drives do.

The interconnect technology used in a storage system or subsystem has a major impact on its performance capabilities. Obviously, the data transfer characteristics of an interconnect have a direct impact on the performance of a device or subsystem. What is less obvious is the fact that the protocol used by an interconnect also has an enormous impact on the performance of the device. SCSI's tagged command queuing (discussed in the preceding chapter) makes an enormous difference in the overall performance of disk drives and subsystems.

Interconnects also directly influence the scalability of a storage subsystem. Obviously an interconnect with a limitation of two devices per controller port cannot scale to the degree of an interconnect that supports hundreds or even thousands of devices.