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

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.



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.

One of parallel SCSI's challenges is the large connector that is located at the back end of the drive and used to connect the hard drive to a server or storage enclosure. The size of the connector will become prohibitive as hard drives shift towards smaller form factors (e.g. 2.5-inch versus 3.5-inch platter size) and because it is unique to parallel SCSI, prevents intermingling of drives within a backplane or enclosure. SAS has a connector similar to SATA's, and consequently can be leveraged to include both SAS and SATA hard drives residing on the same backplane. Cabling for SAS is specified to support lengths of up to eight meters for external attach, in comparison to parallel SCSI that can support a maximum of 12 meters total length for all cables (i.e., storage boxes) connected to the bus. Figure 3 shows how SAS allows a user to "daisy chain" systems as long as the eight-meter maximum cable length is not violated.

The cabling system for SAS improved upon its predecessors by shrinking in size, causing less congestion inside the system enclosure, thus allowing for better airflow. Signaling its enterprise-class roots, the SAS standard employs higher transmit (Tx) differential voltages than SATA in order to drive signals over backplanes and through cables. SAS cabling supports lengths of up to one meter inside the system, such as in workstation and low-end server applications. It also supports cable lengths of up to eight meters for external connections. External SAS connections for enterprise applications are based on the cable developed for Infiniband, which provides four separate links, thereby increasing the overall available bandwidth of up to 1.2GB/sec in each direction (equivalent to 300MB/sec each direction times four links, equals 1.2GB/sec).

Signal integrity is another area of focus since SAS high data rates require that the signal must be exceptionally clean and free of noise. If noise is substantial, it may degrade performance of the hard drive. An example of this would be a noisy signal that prevents the hard drive from acknowledging a specific command. This signal integrity requisite is a result of the high data rate of 3Gb/sec that occurs in both directions. To preserve signal integrity, SAS system cabling must adhere to low impedance requirements and be able to isolate the signal from any extraneous noise and electrical interference that may lead to increased bit-error rates or link errors.

Quality Expectations

The industry expects the overall quality of SAS hard drives to meet or exceed that of parallel SCSI drives. First, the SAS drive mechanical design has leveraged its construction from that of SCSI hard drives, except for the connector, which has been leveraged from SATA drives and enhanced to provide port redundancy. If one of the two ports fails or loses the signal, the other port is still able to communicate to the expander or RAID controller--or vice versa. Secondly, SAS incorporates much better error handling and drive management functionality, as compared to both SATA and parallel SCSI. Finally, because SAS is a point-to-point interface, a failed hard drive is isolated from the rest of the system and consequently, will not cause the entire data bus to go down. All of these features should lead to better overall solution robustness and data integrity.

Hardware Reliability

Reliability of SAS hard drives is expected to be on a par with or exceed that of parallel SCSI, considering that many of the internal mechanical components are shared between the two technologies, such as motor, bearings, and media platter. However, SAS drives will include a dual-port connector which means that even if one port fails, the other port allows the drive to communicate with the expander. Additionally the SCSI protocol supports multi-initiators allowing multiple servers to simultaneously address the same drive representing a true "cluster" environment.

Currently, SAS hard drive manufacturers have reported "theoretical" mean-time-between-failures (MTBF) of 1.4 million hours at 100% duty cycle (24X7). This value directly coincides with the newest parallel SCSI drives that are reporting the same MTBF values of 1.4 million hours at 100% duty cycle. Table 1 details the differences in reliability between SATA, parallel SCSI and SAS hard drives--indicating that SAS is expected to be in line with, or better than, parallel SCSI as the most reliable hard drive technology to date.

Error Reporting

Because of the focus on cost efficiency and simplicity of design, SATA hard drives provide limited error reporting. In comparison, parallel SCSI has been providing its users with at least a base level of error logging and reporting. Going forward, SAS will have the ability through SMP and SSP to dynamically provide the user with detailed error logs, protocol violations, user interventions, and timeout provisions. This information can then be acted upon by the user in "real-time" as opposed to shutting a system down to access the error logs. These errors can now be more accurately and appropriately addressed through management application software. SMP also allows the user to more effectively manage the overall functionality of a SAS system by providing reports of device status and errors.


Thermal and Mechanical Robustness

Thermal stability and mechanical robustness have always been part of the tradeoff between lower cost SATA drives and enterprise-class parallel SCSI drives. This argument will likely still remain with the implementation of SAS drives. The SFF connector--similar to that which is used on SATA hard drives--will accommodate the shrinking size of SAS drives. This advantage will become increasingly important as real estate becomes more valuable in space-constrained storage systems. In addition, the thin cabling for SAS drives helps to improve airflow and is advantageous as systems become even denser and space continues to be a major design constraint.
Device Duty Cycle MTBF (Typical)

Desktop SATA 8 hrs/day, 5 days/week 600,000 hrs
Parallel SCSI 24 hrs/day, 7 days/week 1,200,000 hrs*
SAS 24 hrs/day, 7 days/week 1,400,000 hrs (est.)

* Current generation, next generation for parallel SCSI projected to be
1,400,000 hours

Table 1: MTBF comparisons for different hard drive technologies
(courtesy of Seagate Technology)

Chad Thibodeau is senior advanced quality engineer at Dell Inc. (Round Rock, TX), a member of the SCSI Trade Association.
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Title Annotation:HOT New Technologies; Small Computer System Interface
Author:Thibodeau, Chad
Publication:Computer Technology Review
Geographic Code:1USA
Date:Sep 1, 2004
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