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.
  • 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.
Figure 1. Example of the SAS subsystem
A controller is connected to two expander units in the SAS enclosure.

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

This controller has eight SAS physical link connections. Four of those physical links are connected into two different wide ports. (One connector contains four physical links grouped into two ports; the connectors signify a physical wire connection.) The four-physical link 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 physical link 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.