Using 6Gb/s SAS to Build Flexible Systems

SAS/SATA Compatibility. It’s been a goal of the Serial Attached SCSI (SAS) technology since its inception. Originally this meant that a Serial ATA (SATA) drive could be plugged into a SAS port and work well. But that was back in the 3Gb/s SAS days.

By Susan Bobholz
SCSI Trade Association Treasurer, Server Storage Product Marketing Manager, Intel

SAS/SATA Compatibility. It’s been a goal of the Serial Attached SCSI (SAS) technology since its inception. Originally this meant that a Serial ATA (SATA) drive could be plugged into a SAS port and work well. But that was back in the 3Gb/s SAS days. SAS has matured since then, as has SATA. In addition SSDs are now available. Does 6Gb/s SAS still support compatibility with all these devices?

The answer is a resounding Yes!

Let’s start with a quick review of how SAS supports SATA drives. The SAS connector was originally designed with a keying area. As seen in Figure 1, the keying area on the SATA drive is open and on the SAS drive it is closed or filled in. One reason for filling in the key is that this is where the second port (dual-porting) of the SAS drive is connected. The SAS backplane connector is made with the keying area open so that it will accept either a SATA or SAS drive. Once the specific drive is plugged into the connector, the SAS I/O controller recognizes the type of drive that is connected and sends the appropriate commands to that drive.

Figure 1 – SATA (top) and SAS (bottom) hard drive connectors

It is important to note that a SATA backplane connector fills in the keying area. This means a SATA drive can be inserted into a backplane but the filled in keying area prevents the SAS drive from connecting.

The SAS technology took a leap forward last year when 6Gb/s SAS was launched. Among other new capabilities and features, the speed of the interface was doubled from 3Gb/s to 6Gb/s. Systems and hard drives based on 6Gb/s SAS are shipping today. A key tenant of 6Gb/s SAS is backwards compatibility with the previous generation of SAS. This means that if an older, 3Gb/s SAS hard drive is connected to a 6Gb/s SAS port, the older hard drive will function properly. It is possible to mix 3Gb/s SAS hard drives in the same system as 6Gb/s SAS hard drives and have each hard drive operate at its specified speed.

In parallel, SATA has also moved forward, from the original 1.5 Gb/s to 3Gb/s to 6Gb/s. SATA hard drives running at 6Gb/s are just starting to ship today from several vendors. When 6Gb/s SAS was defined, the engineers and architects in the T10 Technical Committee of INCITS developing the SAS 2.0 specification worked to ensure that compatibility between SAS and SATA was maintained as both technologies moved forward toward 6Gb/s. This was especially demanding as both specifications were being developed at the same time. But the T10 members met this challenging goal. As a result, a 6Gb/s SAS port on a backplane will properly recognize all these different types of hard drives: 1.5 Gb/s SATA, 3.0 Gb/s SATA, 6.0 Gb/s SATA, 3.0 Gb/s SAS and 6.0 Gb/s SAS.

Solid State Drives, commonly known as SSDs, emerged onto the market in the years since SAS was originally defined. SSDs replace the mechanical components in hard drives with flash. SSDs are typically faster than traditional hard drives and anticipated to have higher reliability since they do not contain mechanical components that could wear out or break, such as heads, platters or motors. SSD capacity is much smaller than traditional hard drives and at this time they cost more than hard drives.

What is interesting about SSDs is that they look and feel like 2.5″ hard drives. SSDs were developed to be compatible with the same interfaces used by hard drives – SATA and SAS. An SSD uses the same connector as hard drives and even uses the same protocol as hard drives. Figure 2 shows an SSD where you can see the connector is the same as the connector on a hard drive.

Figure 2 – Example of an SSD showing the connector

This means that today’s SATA SSDs will be able to plug into a 6Gb/s SAS backplane and work. In most cases, no changes need to be made to the system to support SSDs. As SAS SSDs ship in volume, they will also be able to plug into a 6Gb/s SAS backplane and function properly.

Why is it so important for 6Gb/s SAS systems to support all these different types of drives? It enables flexibility. An OEM can build a single board / backplane and when the systems ship, the drive bays can be populated with any of these drives. This reduces the number of boards and backplane skus that must be developed, marketed and maintained in inventory. A VAR or system integrator is able to easily customize a system for a customer simply by installing the preferred drives. Overall, this flexibility reduces costs, which benefits everyone, including the end user.

So what drive is best to use? There’s no single answer to that question. It depends on the specific system and how it’s being used. It’s a balance of performance, capacity, speed, reliability and cost. Look for the balance that works best in your application. Always be sure to check with your system vendor to ensure a specific drive has been tested with your system. OEMs expend significant effort validating drives with their systems and the best experience can be obtained by using a drive they have already tested.

What’s important to remember is that the 6Gb/s SAS port on your system or backplane should be able to support a wide variety of drives. Traditional hard drives or SSDs, SAS drives or SATA drives, 3Gb/s or 6Gb/s should all work and work well, enabling new levels of flexibility never before available with a hard drive interface.

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