Serial Attached SCSI (SAS)
Serial Attached SCSI (SAS) is the latest evolution of the SCSI interface for moving blocks of data between host computers and storage devices in a high performance, enterprise class environment.
SAS overcomes the limitations of parallel SCSI (P-SCSI), which limited the number of devices that could be attached in a single SCSI “domain”, and limited the number of communications that could be occurring at the same time. The speed of the interface is also increasing, so that today’s SAS disk drives can move information across the interface at up to approximately 600 MB/sec (Megabytes per second).
Benefits: As mentioned above, a big benefit to using Serial Attached SCSI is that you can have many more devices in a domain (group of devices that can communicate). Configured properly (using one or more “SAS Expanders”), there can be up to 128 devices attached to a single “device set”, and a domain can have up to 128 device sets, for a total of 16,384 devices in a domain.
Another big benefit is that the SAS interface is compatible with Serial ATA (SATA) drives. That is, SAS and SATA drives can be combined in a storage system to provide the benefits of each type of drive. SAS drives are used for high performance, and SATA drives are used for high capacity.
Protocols: There are three protocols that can be used in a Serial Attached SCSI system:
- SSP (Serial SCSI Protocol) – This protocol is used to send SCSI commands, move data, and get responses from SAS devices. This is considered the “normal” protocol for doing work in a SAS world.
- STP (SATA Tunnel Protocol) – This protocol is used to communicate with SATA devices that are attached to a SAS domain. Virtually all SAS host systems understand this protocol and how to use it. Just like SAS, STP is used to carry commands and move blocks of data back and forth between the host and the SATA devices.
- SMP (Serial Management Protocol) – This protocol is used to perform “discovery” and to control system configuration. Commands are issued by an initiator and are received by an Expander, which responds accordingly.
SAS Expanders: The connectivity and compatibility mentioned above is brought about by the use of SAS Expanders. An Expander is an Integrated Circuit (IC or Chip) that usually resides on a SAS backplane, and they have a port count that ranges from 24 to 36 ports (today). These ports can be configured to communicate with devices across the backplane, host bus adapters (HBAs), or other expanders in the system.
To oversimplify, an expander acts as a switch that allows connections to be made between host computers and storage devices. But they do more than just switching, as they can act as a “bridge” between a SAS HBA and a SATA device, using the STP protocol (see above). The expanders are also the part of the system that allow devices to be formed into groups, or zones, so that only certain host systems can see a limited number of devices. This allows large storage systems to be divided and shared among many hosts.