SCSI finally gains serial attachment [SAS] … after decades of steady progress

Computer Technology Review, Sept, 2004 by Chad Thibodeau

The current generation of Small Computer System Interface (SCSI) hard drives reached its physical speed threshold at 320 MB/sec, stimulating the evolution to a new serial-based technology for hard drives, Serial Attached SCSI (SAS). SAS technology extends the benefits of parallel SCSI, such as high reliability, enterprise-class performance and manageability, while also addressing the expected increase in I/O and storage requirements that are a direct result of high-availability and high-performance clustering applications.

SAS hard drives will initially operate at 3Gb/sec, duplex--translates to a data transfer rate of 300MB/sec in each direction. The SAS roadmap takes the data transfer rate to 6Gb/sec in its next release and to 12Gb/sec in 2012. Meanwhile, Serial ATA (SATA) is also expected to increase in speed and will continue to be the low-cost, high-density solution for consumer and low-end enterprise products. Figure 1 shows the evolution of the SCSI/SAS interface in comparison to the number of disk drives in an array.

Most parallel I/O technologies are migrating from parallel to serial. Examples of this migration include: PCI-x to PCI-e, Parallel ATA to Serial ATA, and parallel SCSI to SAS. The migration is not due to the method of transfer, but to the evolving server and storage solution speed requirements and the amount of bandwidth needed to manage the ever-increasing volume of data. This increasing amount of bandwidth comes at a price that parallel users must take into account--both the expense of the newer technology and the complexity that exists when attempting to meet this demand. SAS offers the easiest, most straightforward storage system migration.

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SAS Advantages

Benefits of SAS hard-drive technology include:

** Uses SCSI commands to preserve the value of parallel SCSI investments

** Compatibility with SATA for deployment of SATA drives in SAS system

** Serial transmission for higher data transfer (3Gb/s today, 12Gb/s tomorrow)

** Point-to-point architecture for highly available bandwidth and scalability

** Full duplex, dual-port design for faster throughput and greater uptime

** Improved addressing over parallel SCSI (single SAS domain is 16,384 devices)

** Compact cabling and connectors for simplified cable routing and improved airflow

** Enterprise-proven components and reliability for mission-critical applications

As a complement to parallel SCSI hard drives, SAS hard drives promise to bring the reliability and proven enterprise-class performance of parallel SCSI, with the scalability and manageability of serial point-to-point interface technology. The driving force behind the SAS standard was to develop the hard drive interface to be seamlessly compatible with both existing SATA and new SAS hard drives--thereby allowing a user to mix SAS and SATA drives in the same system for greater flexibility in deploying storage solutions.

In addition, SAS will highly leverage the proven mechanical technologies of SCSI drives, as well as use the same command set as parallel SCSI, while enhancing device addressability and bandwidth scalability. SAS also promises to provide universal connectivity with SATA hard drives. In an effort to increase the number of devices from 16 that could exist in a SCSI domain, SAS has the ability to attach up to 128 SAS storage devices per edge expander module, multiplied by 128 devices per fan-out expander module (see Figure 3). This will greatly increase the potential array size to 16,384 devices in a single SAS domain.

New Protocols

With the development of the Serial Attached SCSI standard came three new protocols, or means of communicating between hard drive devices and host initiator devices, management software and system enclosures. Serial SCSI Protocol (SSP) is used to transfer SCSI commands; SAS Management Protocol (SMP) is used to send expander device management information, manage and configure expanders; and Serial ATA Tunneled Protocol (STP) is used to transmit SATA commands. The SAS expander permits the host to communicate with either SAS or SATA drives. Of the three, SMP is the most interesting, yet simplest protocol, since it provides overall status management of the SAS infrastructure.

Connectors and Cabling

When the SCSI Trade Association conceptualized the marketing requirements for SAS, they made every effort to leverage the benefits of both parallel SCSI and SATA. The major area of leverage was to base SAS drives on the proven technology, robust drive mechanics and time-tested command interface of parallel SCSI drives. The SAS standard also builds upon the connector and cabling system utilized by SATA drives. SAS drives use a SFF (formerly Small Form Factor) Committee-designed connector that is compatible with the SATA drive connector, such that it is possible to plug a SATA hard drive into a SAS cable or backplane, but not vice versa. This compatibility allows for greater flexibility for customers' storage configurations.

In addition, the connector design for SAS hard drives is similar for both backplane and cable applications, in comparison to parallel SCSI which has entirely different connectors depending on whether it will be connected to a backplane or cable. In order to improve the retention force of the connector/backplane or connector/cable interface, the SFF Committee is currently investigating the adoption of a similar latching connector for SAS hard drives, as will be used on SATA hard drives beginning in Q4 of 2004. This latching connector design would help reduce the connectivity problems that have affected previous generation drive technologies. Finally, the SAS connector, similar to its SATA counterpart, fully supports hot plugging and blind mating when connected to a backplane. Figure 2 shows a comparison between a SATA connector, parallel SCSI connector and the proposed SAS connector.