Tech: Serial-Attached SCSI Overview Notes (Part I)

SAS is the latest in storage technology. First there was IDE, then ATAPI, then SCSI and Fibre-Channel, then SATA. SAS is backward compatible with SATA and implements SCSI commands.

• Devices connected together using phys. A phy is a connection to a physical link.
• Port = group of one or more phys that share a common address. A port can be connected to multiple other ports as physical links will allow.
• Protocols:
• SSP = Serial SCSI Protocol -> communication between initiator and SAS device.
• STP = Serial Tunneling Protocol -> communication between SAS host and SATA target.
• SMP = Serial Management Protocol -> management of expanders and the service delivery subsystem.
• Expanders can host up to 128 devices, but can also be cascaded to support up to 16385 devices.
• Device = initiator, target, expander, or anything else that can be addressed (including virtually).
• Expander routing:
• Direct = when device is directly attached to expander.
• Table = when device is known to expander via one of its phys/ports, not direct-attach.
• Subtractive = not directly attached or routing table missing/no entry.
• Types of expanders:
• Fanout = addresses of all devices in the domain known to expander. Uses direct or table routing, no subtractive routing.
• Edge = devices known to expander are direct-attach only. Uses subtractive routing for unknown addresses, table routing or direct addressing for knowns. Can have 0 or 1 subtractive ports.
  
• Edge expander device sets = combination of fanout and edge working as one unit; follows edge expander rules.
• Domain = contains all devices that can directly interface; logical representation of devices.
  
• An expander without zoning can only exist in one domain; devices with multiple ports can exist in more than one domain.