SAS architecture summary

Elements that interact to enable the structure of the SAS architecture include controllers, ports, and expanders.

The following points are applicable to this description of general SAS architecture:

• A SAS fabric describes all possible paths between all SAS controllers and I/O devices including cables, enclosures and expanders.
• A SAS controller, expander and I/O device contains one or more SAS ports.
• A SAS port contains one or more physical links (phys).
• A SAS path is a logical connection between a SAS controller port and I/O device ports.
• SAS devices use the SCSI command set and SATA devices use the ATA/ATAPI command set.

The example of a SAS subsystem in the preceding figure illustrates some general concepts.

This controller has eight SAS phys connections. Four of those phys are connected into two different wide ports. (One connector contains four phys grouped into two ports; the connectors signify a physical wire connection.) The four-phys connector can contain between one and four ports depending on the type of cabling used.

The uppermost port in the figure shows a controller wide port number 6 consisting of phy numbers 6 and 7. Port 6 connects to an expander which attaches to one of the I/O devices dual ports.

The dashed red line indicates a path between the controller and an I/O device. There is another path from the controller's port number 4 to the other port of the I/O device. These two paths provide two different possible connections for increased reliability by using redundant controller ports, expanders and I/O device ports. The SCSI Enclosure Services (SES) is a component of each expander.