An Introduction to SAS

Author: Susan Bobholz, Market Development Manager

Serial Attached SCSI (SAS) is the logical evolution of the traditional parallel SCSI interface, and it represents the continuation of more than 25 years of technology development and infrastructure investment. In SAS, the SCSI protocol is transported over a serial interface. Compared to the existing parallel SCSI interface, SAS enables faster device interconnect speeds, simpler cabling and improved system reliability, while preserving existing SCSI capabilities. SAS also improves connectivity to larger numbers of drives and also enables compatibility with Serial ATA (SATA) drives. The SAS interface standard was developed by the T10 Technical Committee of the International Committee for Information Technology Standards (INCITS). The latest specification “Serial Attached SCSI version 1.1” has been adopted by the American National Standards Institute (ANSI). A wide variety of SAS products has been shipping for several years including hard drives, controllers, servers and networked storage systems.

What is SAS?
SAS is the high-speed serial successor to Ultra320 SCSI as well as an I/O infrastructure that provides for a wide range of storage solutions. SAS goes beyond a mere disk drive interface; it provides a set of components that include connectors, cables, and expanders that can be used to deploy a large number of solutions into the storage and server market. While SAS preserves the traditional SCSI usage model, it widens the types of applications and markets it can serve including blade storage, tiered storage and near-line storage, and shared storage between hosts.

The Objectives for Developing SAS
The main objective for developing SAS was to preserve as much legacy SCSI software as possible. There are over 25 years of middleware, firmware, and drivers installed in the market, which translates into a very large volume of base information and expertise that needed to be preserved.

The second objective was to allow choice between low-cost, high-capacity SATA drives and high performance/highly reliable enterprise class SAS drives within the same infrastructure. This enables tiered storage solutions especially in the areas of archiving or replication, as well as the ability to service high availability applications all with the same technology.

A third objective was to preserve the economics of the base architecture in the server. In other words, it was vitally important to maintain the cost point of any SAS implementation with any previous Ultra320 SCSI implementation models.

The forth objective was to maintain equal or better performance characteristics of previous SCSI designs to differentiate a SAS-class system from an ATA-class system. In other words it was critical that SAS continue to deliver superior performance especially within the server environment.

The fifth objective was to provide scalability beyond previous parallel SCSI limitations by having the ability to service thousands of connections. At the time SAS was first being developed, small form factor disks (for example, 2.5-inch) were being developed. These small disks enable a significantly larger number of disks in the same physical space and thus SAS needed to support significantly larger numbers of connections.

Finally with the requirement to accommodate more and more devices on the bus, SAS needed to pay attention to system reliability. SAS addresses reliability by defining dual ports on the disk drives to enable fail-over, and by employing a point-to-point architecture which provides fault isolation.

Advantages of SAS
The advantages of Serial Attached SCSI over traditional parallel SCSI are numerous. This section discusses some of those advantages.

Enhanced System Performance: SAS is a full-duplex connection which means there is a path in each direction that simultaneously runs at full speed. Parallel SCSI is a bi-directional bus where commands are issued in one direction at a time; the bus is then stopped and turned around so the targeted device can reply in the other direction on the same bus.

Improved Air Flow: Because SAS utilizes four wires instead of 50 to 68 wires, SAS cabling is much thinner. This reduces airflow constraints in system enclosures. Figure 1 shows both a SAS/SATA cable and a parallel ATA cable illustrating the size difference.


Figure 1: Serial and Parallel Cables


Simplified Cabling: SAS cabling employs point-to-point architecture instead of a bus so each device has its own cable.

No Terminators: Termination is built into the SAS receiver which eliminates the need for external terminators.

SAS Capabilities
SAS delivers new capabilities that extend well beyond those found in parallel SCSI. Some of these new capabilities are described below.

Ability to Address Thousands of Devices: SAS defines the ability to address thousands of devices which is considerably more than parallel SCSI. Worldwide names instead of jumpers are used to define addresses, thus eliminating the duplicate addresses that often occur in parallel SCSI.

Improved Density: SAS defines a Mini SAS 4x connector shown in Figure 2. The density characteristic of the Mini SAS 4x connector enables up to four external connections on a standard height PC card. With four external connectors and four independent links in each cable, this means that up to 16 individual links are available to the system. There is an internal equivalent to the Mini SAS 4x scheme which is called the Mini SAS 4i. The Mini SAS 4i provides significant scaling capabilities within the server or JBOD/RAID enclosure.


Figure 2: Mini SAS 4x Connector


Bandwidth Aggregation: A SAS wide-port enables four SAS links to share a single connector thus aggregating signals. In SAS, a link operates at 3Gb/s in each direction at full duplex, enabling a 6Gb/s link. With four links sharing a single connector, a system is able to obtain up to 24Gb/s across a wide port. Typically, servers and storage solutions utilize one of two port configurations. The first is a two-port configuration and the second is often a four-port configuration, depending on the manufacturer. Server and storage solutions will thus have either 12 or 24Gb/s within a single cable. With parallel SCSI, the system would need a dozen or more SCSI buses to achieve the same throughput. This concurrency delivers very high performance because a single cable can have multiple simultaneous operations to multiple devices. In addition, waiting and bus contention issues found in large parallel SCSI systems are significantly reduced, thus improving overall system performance even more.

Blind Mating and Hot Plugging: SAS was designed so that disk drives could be easily inserted into and removed from an enclosure backplane. Each connector has the data transmit and receive signals on the left side of the connector and device power connections on the right side of the connector. Also important is the keying in the middle of the connector. The pins are staggered to provide staging of power, thus enabling the hot-plug ability of the connector. There is also a set of pre-charge pins to assist the enclosures power source when hot plugging devices so that they do not experience a significant current load when drives are added to the enclosure. The connector includes slots at the ends of the connector that allows the device to be blind-mated into the enclosures backplane. As a result, system administrators needn’t be able to see inside the enclosures when adding or removing devices.

SAS / SATA Compatibility: SAS natively supports the ability to plug either a SAS drive or a SATA drive into the same backplane slot. This ensures the flexibility to decide which drive best serves each application. Systems can be preconfigured or they can be easily reconfigured in the field as requirements for systems and operations change. SAS / SATA compatibility is accomplished with a keying area. As seen in Figure 3, the keying area on the SATA drive is open and on the SAS drive it is closed or filled in. One reason for this key filling is to facilitate configuration of the second port (dual-porting) of the SAS drive. The SAS backplane connector is made such that it will accept either a SATA or SAS drive. SATA drives are single-port, hence the keying is open on a SATA drive and no connection is made to the Port B part of the connector.


Figure 3: SATA and SAS Hard Drive Connectors


It is important to note that a SATA drive can be inserted into a SAS infrastructure and backplane, but a SAS drive cannot be plugged into a SATA backplane connector.

Serial Attached SCSI is the logical extension of parallel SCSI, extending a 25-year tradition of technology and infrastructure investment. SAS provides new capabilities that are not available in parallel SCSI, including the ability to address thousands of drives, hot-plugging, and compatibility with SATA drives. These new capabilities enable system vendors and resellers to offer unprecedented flexibility and performance to their customers.

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