Susan Bobholz, Product Marketing Manager, Intel
With Contributions by Harry Mason, Director, Industry Marketing, LSI Corp. and Jay Neer, Advanced Technical Marketing Manager, Industry Standards, Molex

Serial Attached SCSI (SAS) burst upon the storage scene in 2004 as a serial replacement for the once ubiquitous parallel SCSI interconnect. While much attention is often given to the capabilities of the protocol and to the silicon and software required to implement storage systems, it is often the physical interconnections that influence the adoption of I/O in a variety of market segments. Parallel SCSI, for example, supported everything necessary for clustered server deployment, however, the large unwieldy nature of parallel cables and the restricted cabling distances greatly limited SCSI's applicability in these markets.

SAS removed many of these restrictions by opening up the bandwidth, improving cabling distances and providing a compact connection scheme suitable for today's shrinking data center. While first generation solutions made tremendous strides as the de facto DAS replacement, connectivity has continued to improve, opening up new markets for SAS in the process.

This article is the first in a series of 3 articles discussing SAS connectivity. This article introduces the basic concepts of how SAS devices connect together as well as how SATA devices connect into a SAS infrastructure. Article #2 discusses how SAS connectivity has evolved over time, while Article #3 discusses the future of SAS connectivity and the Advanced Connectivity Roadmap. Please read all three articles to understand the complete SAS connectivity story.

Introduction to SAS connectivity

At the heart of SAS connectivity is the connector. The SAS hard drive connector was originally designed to be compatible with the SATA hard drive connector since an original goal of SAS was to allow either a SAS hard drive or a SATA hard drive to be installed into a SAS system. This provided unprecedented choice for the OEM or system builder to install the type of drive that best fit the application and usage of a particular system.

SAS devices are capable of connecting directly to other SAS devices or to SATA devices via one of three methods:

• Backplane Connectors
• Internal Cables
• External Cables

Backplane Connections

Whether it's on the motherboard, within a rack of disk drives, in an external JBOD or a removable storage blade, enterprise storage is designed to accommodate multiple drive connections, which can be inserted or removed without interrupting system operation. To support this dynamic, drives are frequently connected directly to the backplane within a system or within a drive enclosure.

Figure 1 shows the drive connector used for SATA and SAS drives. They are very similar, with the notched portion of the connector being the most noticeable difference. A SATA drive has the notched portion open while a SAS drive connector fills in the notch. A SAS backplane connect will accept either a SAS or a SATA drive because the 'notch' area of the backplane connector is open. However, a SATA backplane connector fills in that notch. So this filled-in notch prevents a SAS drive, which also has a filled-in notch, from being inadvertently plugged into a SATA-only system.

Figure 1: The top connector is a typical SATA Disk Drive connector (Serial ATA, Rev 3.0) while the lower connector is a typical SAS Disk Drive connector (SFF-8482).

In a backplane environment these drives are typically mounted in a drive carrier and can be dynamically inserted into (hot-pluggable) or removed from the SAS backplane connector (see Figure 2).

Figure 2: This is a typical backplane connector that shows how a SATA or SAS drive mates to the system connector (SFF-8482).

Cabled Connections

Another option is to connect SAS devices with cables instead of using a backplane. Cabled connections span a wide range of needs for both OEMs and system builders. In addition to unshielded internal cables, connectivity options include externally shielded connections, drive-to-controller connections, drive-to-backplane connections and host-to-drive connections.

Internally, a multi-port SAS controller is connected to the system board and cabled to individual drives as either SATA or SAS connections. Figure 3 shows a simple SATA-style connection using the 7-pin SATA/SAS connector from a single port. The remaining portion of the cable shown provides power to the Hard Disk Drives (HDDs). This cable style may look very familiar as this style is also used in desktop or client PCs to connect SATA drives to the SATA controller. In all cases, this cable connects one drive to one port.

Figure 4 shows how an "octopus" cable may be used in multiple drive configurations. These cables connected four SAS ports on a board to 4 drives, using a much smaller footprint on the board.

Figure 3: A typical 7-Pin SATA/SAS type connector carrying a single SATA/SAS port.

Figure 4: SAS cables generally originate from a single SFF-8484 connector and fan out into four SFF-8482 connectors. (These are often referred to as Octopus cables and are useful for internal connections). (Courtesy Molex)

Large system configurations need to connect externally and different cables are needed for external connections due to the shielding requirements. Early SAS products that needed to expand externally to a JBOD or an external RAID enclosure used the SFF-8470 InfiniBand-style connector. This connector was shielded for external high-speed signaling and was capable of carrying four SAS ports down a single cable (See Figure 5).

Figure 5: SFF-8470 InfiniBand-style connector supports four SAS ports. (Courtesy Amphenol)

SAS Connections Evolve

The InfiniBand-style connector worked well for many externally connected SAS devices. However the industry desired similar connectors for both external and internal use and also desired a method of connecting more ports along the edge of a circuit board.

The Mini-SAS alternative emerged early in the SAS adoption cycle and has become the mainstream connector for both internal and external SAS connections. Like the InfiniBand-style connectors, the SFF-8087 (see Figure 6 - internal) and SFF-8088 (Figure 7 - external) Mini-SAS variants offer four SAS channels per connection, but at twice the density. The Mini-SAS connector not only has a substantially smaller footprint, but also allows more ports to be placed along the edge of a printed circuit board. The Mini-SAS connector became a mandatory requirement for 6Gb/s SAS compliance, which has greatly contributed to its overall popularity.

Figure 6: SFF-8087 Mini-SAS 4i (used for internal SAS and SATA connections, four channels per connector). This figure shows various adaptations of this standard connector with alternative mating options.

Figure 7: SFF-8088 Mini-SAS 4x (used for external SAS and SATA connections with four channels per connector). Various options for board and card assemblies are widely available.

New Connectors for SSDs

SSDs are emerging in the enterprise as an alternative to hard drives. SSDs implement SAS and SATA protocols and use the same connectors as hard drives and connect the same way as hard drives. However, some SSDs are appearing in a 1.8" form factor and need a different connector. A new adaptation of the SFF-8482 connector has emerged for 1.8" drive form factors. This form factor is growing in significance for certain classes of Solid-State Disk (SSD) drives. Figure 8 illustrates what the industry refers to as the Micro-SAS connector.

Figure 8: Micro-SAS (SFF-8486) connector. (Courtesy Molex)

Conclusion

From its earliest concepts, connectivity has always been a key component of SAS, enabling system configurations never before possible. With SAS, it is possible to connect either a SAS drive or a SATA drive to a SAS system. Both back plane and cabled connections are available, with cabled connections available for use internal to the system and externally to JBODs and external RAID arrays.

The industry continues to change and evolve and SAS has kept pace. As SAS has matured, the industry has settled on using the Mini-SAS connect scheme for both internal and external connections. In addition, as 1.8" SSDs have emerged, connectors have been defined to enable those SAS and SATA devices to connect into a SAS system.

Connectivity is a key component of SAS as it is critical that all the devices used in a SAS system connect together in a standard way.

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