Serial Attached SCSI


Definition: Serial Attached SCSI (SAS) is a new generation hard disk that is deployed for high speed data transfer between devices such as hard drives and CD-ROMs.

In a serial transfer of data, bits of data move linearly in a single path. In a parallel mode, multiple bits of data crowd the pathway thereby affecting throughput efficiency. Bandwidth utilization is optimum when data is transferred serially. Unlike a parallel transfer where there is competition between bits of data that start out together but do not arrive simultaneously due to variable gating and buffer delays and varying signal path lengths, a serial transfer is smooth. Data integrity is preserved on serial lines.

Speedy information transfer is the need of the hour in this day and age of fast connectivity and the dependence of organizations on current data to take the right decisions at the right time. The importance of having the right storage and retrieval systems in a data centre is growing by the day. Serial Attached SCSI offers the possibility of data transfer at gigabit speeds. It is cheaper than Fibre Channel and sturdier than SATA.

The Business Proposition

Serial Attached SCSI (SAS) has made everybody from vendors, users, administrators, and CIOs to sit up and take notice. SAS incorporates in it features of SCSI, SATA, and Fibre Channel technologies. This makes it a storage device that can fit into most storage environments and other computing environments such as servers and workstations. SAS can theoretically scale to 4032 devices per port which means increased opportunity for deployment in commercial and technical applications.

SAS also provides excellent support for legacy SCSI software which means that migration to SAS is cost-effective and smooth. Apart from excellent backward compatibility, another thing that SAS has going in its favour is the development of standards and protocols at an early stage. These protocols include:


SAS Topologies
1. The Serial SCSI Protocol for managing extant SCSI software

2. The Serial Management Protocol for managing new point-to-point connectivity

3. The SATA Tunnelling Protocol that lays down the rules for interconnectivity between SATA devices

SAS allows users to choose between low-cost SATA drives for storage of routine data and high performance SCSI drives for mission-critical data and applications. The cost of deployment is determined by the final application. The SAS backplane offers the solution provider and the end user a choice of drivers which means that storage configuration can be done in the most effective manner for a given application. The serial point-to-point connection offered by SAS gives the kind of performance that storage systems expect.

SAS has at its disposal the experience acquired over 20 years in the development and running of parallel SCSI, which at present covers the majority of the enterprise disk market. The entry of SAS offers a robust and cheap alternative that meets the demand for heavy-I/O applications such as databases and web servers.

The advantages of SAS over SATA drives include provision for speeds up to 3 GB/s as against 1.5 GB/s in the current generation of SATA drives. The next speed increase in SAS is slotted at 6 Gb/s. SAS features a full duplex architecture that enables simultaneous bi-directional data and command transfer; SATA drives operating at 7,200 rpm cannot match the capacity and performance of SAS drives running at 15K and 10K rpm; A single fast high-speed port is formed by linking multiple high-speed physical links as a result of the wide port capabilities that SAS features.

SAS also offers dynamic load balancing. This allows all theSAS Hard Disks - SAS Connectors on Seagate 10K 73Gb 2.5in. & Fujitsu 15K 73Gb 3.5in Hard Drives controllers to take part in processing the I/O requests. Overload on one controller and a skew in I/O requests is thus avoided. SATA does not possess dynamic load balancing capability.

Concrete benefits of SAS:

Interoperability: Interoperability has been a key feature of SAS ever since its advent in 2003. It enables compatibility between SAS products developed by different vendors thereby reducing the end-userís dependence on a given vendor.

SAS offers high performance, for example a single SAS connection supports 4 SAS wide lanes or 4 x 300 MB/s per connection. Theoretically, a speed of 1,200 MB/s can be supported with a SAS connection on a JBOD system.

SAS offers high availability of data due to the dual port capability in each SAS drive. Thus, in the event of a controller failover there is an extra path available to each drive. A separate point-to-point connection for each drive ensures that the loss of connection to a drive does not affect all the drives. SAS drives are rugged and built for the stress of enterprise use; they have warranties of up to 5 years and MTBF ratings that exceed 1 million hours.

A Serial Attached SCSI system has the following components:

Initiator: Task management and device service requests originate at the initiator. The device acts as a receiver for responses from target devices regarding these requests. Generally, the initiator is a part of the host computer and peripheral devices are the target. The initiator is available as a part of the motherboard or as an add-on host bus adaptor.

Target: Requests for processing are sent to the target. The target contains logical units and target ports which process the device service and task management requests. An example of a target device would be a hard disk.

Service Delivery Subsystem: The transfer of information between an initiator and a target takes place with the help of a service delivery subsystem which is connected to the initiator and target with cables. Expanders may also be present.

Expanders: These connect multiple SAS End devices to a single initiator port.