SAS – a Sharable, Scalable Storage Technology

Author: Susan Bobholz, Storage Market Development Manager,

Does the storage industry really need another storage fabric? There have been heated debates over the use of Fibre Channel, iSCSI, even InfiniBand, as THE best fabric, that continues even today. Yet, as larger and larger SAS-based topologies are being implemented, there is discussion over using Serial Attached SCSI (SAS) as a fabric technology. To fully understand this phenomenon, it’s important to understand the roots of SAS and how SAS systems are being architected.

SAS was developed as the natural evolution of parallel SCSI, enabling point-to-point drive connections via a serial interface. In its simplest form, one SAS port on a SAS Host Bus Adapter (HBA) connects to one drive. Because SAS drives can be dual-ported, a single SAS drive can connect to two SAS HBAs. This enables redundant systems to be built, such that if one SAS HBA failed, the data on the drive is still accessible via the second HBA.

This is an excellent model for storage inside a server, however in many instances, more storage is needed than will fit into the server chassis. To support direct-attached storage outside of the server, the concept of an expander was defined. SAS Expanders enable a simple switching topology. SAS expanders are often incorporated into SAS Just a Bunch of Disks (JBOD) boxes, enabling a single SAS connection from the server SAS HBA directly to the SAS JBOD. Additionally, SAS expanders allow multiple servers to connect to the same SAS JBOD, as shown in Figure 1. The SAS JBOD can then be shared between multiple servers.


Figure 1: Multiple Servers Sharing a JBOD


Scaling SAS Implementations
One of the benefits of SAS expanders is that they can be cascaded, enabling very large configurations to be built. A single SAS domain may support up to 16,384 devices. Figure 2 shows a relatively simple configuration, with two servers connected to two cascaded JBOD enclosures where each enclosure contains a SAS expander. It would be simple to scale this configuration to support more storage by adding a third JBOD enclosure.


Figure 2: Cascading with SAS Expanders


A large SAS configuration would enable multiple servers to connect to multiple cascaded SAS expanders. Such a configuration could become quite unwieldy because every server does not necessarily need to access every hard drive. The SAS-1.1 specification enables the concept of SAS zoning, whereby a configuration of SAS hard drives is broken into groups, or zones, and servers are enabled to communicate with drives in one or more zones. IT managers are able to specifically prevent some servers from communicating with some zones. SAS zoning is implemented in the SAS expander and this new expander essentially becomes a SAS switch. Once SAS switches are incorporated into a configuration, a SAS fabric has been built.

Comparing a SAS Fabric to Other Storage Fabrics
One reason SAS is gaining popularity as a storage fabric is that it fills a gap left by the other fabric technologies. Table 1 shows how SAS fabrics compare to Fibre Channel, GbE iSCSI and 10 GbE iSCSI fabrics.


Table 1: Differences between Storage Fabric Technologies


As has already been established in the storage community, Fibre Channel enables high-performing fabrics to be built over long distances. Fibre Channel supports connections between devices that are up to several kilometers apart, operating at speeds up to 10Gb/s. However, building a Fibre Channel fabric can be expensive.

iSCSI has become established as an alternative fabric to Fibre Channel. The first iSCSI fabrics were built upon 1Gb/s Ethernet and had the advantage of being significantly less expensive to build and maintain than Fibre Channel. The newest iSCSI fabrics are built upon 10Gb/s Ethernet which significantly improves performance over the first iSCSI fabrics, but also raises the cost.

SAS fills the need for a high-performance, low-cost fabric that is not required to span long distances. In other words, a high performance SAS fabric can be implemented within a single rack, or perhaps within a room at a lower cost than any other high performing storage fabric.

Configurations Underserved by SAN Alternatives
SAS fabrics are being implemented in configurations where other SAN alternatives are not attractive. These are configurations where high performance is needed and the solution needs to be low cost, yet doesn’t need to cover large distances. One example of such a configuration would be a small scale “data center” with less than 50 servers and storage arrays. All this equipment is stored in one room. This configuration needs a high-performing method of connecting the servers to the storage arrays but without the high cost of traditional SAN fabrics, such as Fibre Channel and 10Gb/s iSCSI. A SAS fabric is a good solution, since SAS hard drives are likely already used in the storage array. By using a SAS fabric, the data being written to, and read from the hard drives does not need to be translated to a different protocol to move thru the fabric.

Another example of a configuration that benefits from having a SAS fabric is a server blade configuration – also called clustered servers. Clustered servers are small 1U boxes that do not have room for many (if any) hard drives to connect inside the box. The hard drives are usually in a separate box, sometimes called a storage blade. A rack could be built with multiple server blades connecting to multiple storage blades. In this scenario, a high-performing fabric is needed to connect and control access to the storage, but the overall cost needs to be low. Again, a SAS fabric is a good solution. This would be implemented by incorporating a SAS switch into the rack.

SAS Is Scalable, Sharable Storage
Does the industry really need another fabric technology? The industry has already responded and is saying “Yes”. SAS expanders built into SAS switches enable SAS fabrics to be built. SAS fabrics make it simple to add more storage to a configuration. SAS hard drives in a SAS fabric are sharable to all servers connected to the fabric. SAS zoning allows administrators to divide the storage into segments and then decide which servers are allowed to access which segments. This technology is enabled in the recently released SAS-1.1 specification with products available today.

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