SAS and SATA compatibility: a new paradigm for disk interconnects - Tape/Disk/Optical Storage

Computer Technology Review, June, 2003 by Mike Wall

The relatively staid world of disk interconnects is suddenly heating up. ATA disks are transitioning to Serial ATA (SATA). SCSI disks are going to transition to Serial Attached SCSI (SAS). To make this even more interesting, the SAS technology enables a system to be built that accepts both SAS hard drives and SATA hard drives. This compatibility between SAS and SATA is a completely new paradigm that demands our attention.

Serial Technology Benefits

It has become increasingly difficult for SCSI and ATA to continue to increase performance with their existing parallel buses. The transition from parallel to serial technology is necessary if SCSI and ATA are to meet future requirements for performance and manageability There are many benefits of transitioning SCSI and ATA to serial technology. Table 1 summarizes the features serial technology brings to SCSI and ATA, as well as the benefits of each.

Who Needs SAS/SATA Compatibility?

Who benefits from compatibility between SAS and SATA? Nearly everyone involved in building, selling and using servers and networked storage. Let's look at the different groups that benefit from this breakthrough in more detail.

Figure 1 illustrates how mainstream servers take advantage of this compatibility.

Today, many backplane manufacturers develop a backplane for SCSI drives and a separate backplane for ATA drives. SCSI and ATA require routing a large number of signals that lead to many layers in the backplane. The use of serial technologies reduces the number of layers in the backplane, resulting in a less expensive board. And, the emergence of SAS means that manufacturers need to develop only one backplane. Vendors can use the same SKU to support both SAS and SATA drives.

Compatibility means that system OEMs can now sell an upgrade package simply by selling new drives. This is accomplished by utilizing a backplane that supports both SAS and SATA drives. The use of one backplane simplifies inventory management and results in lower costs and validation efforts.

SAS and SATA compatibility enables VARs and integrators to easily configure a custom system for an individual customer, simply by installing the preferred disks. VARs no longer need to worry about installing the proper backplane, the proper cables and the proper drives. Instead, they can simply populate the backplane with the desired drive type. Overall benefits include reduced inventory costs, easier product differentiation and simplified training.

End users benefit from the cost reductions that SAS/SATA compatibility provides to backplane manufacturers, system OEMs and VARs. The ability to change from SATA to SAS drives without purchasing new systems simplifies the upgrade process and helps future-proof end-user investments. In addition, systems using serial technologies are easier to configure. Users no longer have to contend with termination, jumpers or master/slave configurations.

How SAS/SATA Compatibility Works

Let's investigate how SAS and SATA compatibility is accomplished. There are two aspects of the technology that enable compatibility: Physical/electrical and connector.

SAS and SATA have very similar physical and electrical interfaces. The differences between the two interfaces, are due to the fact that there are different primary usage models for each. For instance, SATA is a technology developed for the desktop and must use spread spectrum clocking to pass FCC "Open Box" emission requirements. SAS is primarily designed for servers and enclosures, which are considered "Closed Box" systems and, thus, are not required to meet the FCC emission requirements. Another difference is the differential voltage ranges. SAS voltage ranges are higher than SATA ranges because servers and enclosures require signals to be driven over backplanes and cables up to 10 meters in length. The SATA voltage ranges enable driving signals over a 1-meter cable. It takes stronger signals to drive 10 meters of cable than 1 meter. In addition, it takes more strength to drive signals over backplanes than it does to drive the signals over a 1-meter cable.

The SAS backplane connector was designed to accept either a SAS drive or a SATA drive, while making provisions for a dual-ported SAS drive. SAS connectors add a notch to the SATA connector to ensure that SAS drives can only be plugged into SAS systems. This is required because SATA host controllers do not understand SCSI protocols and will not recognize a SAS drive. Notching the SAS drive connector prevents a SAS drive from being plugged into a SATA host controller, which would result in a non-functioning drive.

Choosing Between SAS and SATA Drives

A natural question arises from SAS/SATA compatibility: When should a user specify SATA disks, and when should one use SAS disks? The answer is based on the inherent differences between SCSI drives and ATA drives today.

SCSI disks are designed and manufactured to meet the enterprise requirements of high reliability and high mean-time-between-failure (MTBF). The MTBF in SCSI drives is measured over continuous usage 24x7 and 52 weeks per year. The high rotational speeds of SCSI drives (10K and 15K RPM) and low seek times result in very high performing drives. SCSI drives are tailored to support mission-critical data in applications where reliability and availability are key requirements. SAS disks will continue this trend.