Is SAS a Fabric?

Martin Czekalski, Interface Architecture Initiatives Manager
Maxtor Corporation

Well, with a variation of that now-classic line, it all depends on what your definition of “fabric” is. If your definition includes a large computer room, campus infrastructure environments or generalized network capabilities, then the answer is “No.” For example, if you have multiple computer rooms full of servers and storage area network (SAN)-attached storage with the need to connect a cross-campus environment, then either a Fibre Channel (FC) or IP-based SAN is going to be the obvious choice. These protocols and interconnects allow for a high degree of connectivity as well as long cabling distances. However, many applications that currently use SANs are much more bounded, and could easily be satisfied by using Serial Attached SCSI (SAS). Let’s look at some usage models and see how SAS can provide many of the same features as a SAN in these bounded environments.

One of the features that people use to differentiate a fabric from a simple interconnect is the ability to perform routing. SAS is a connection-based protocol and hence does not route on a packet-by-packet basis. However, SAS does use a form of routing to establish a connection with the OPEN_ADDRESS frame. The path taken to a device can change, as the result of system reconfigurations, but the contents of the OPEN_ADDRESS frame that initiates the connection remains the same. Routing of this frame occurs in the expanders by the use of either a subtractive or table route method. Based on this model, many people would agree that SAS is a fabric.

One of the major differences between SAS and existing SAN fabrics is the distance that can be spanned. For example, SAS cables can typically be extended up to six meters, while FC can be extended to 10 kilometers with the use of single-mode fibre. While SAS could be theoretically extended to a longer distance by using optical connections, to date there are no standards defined to do so. Additionally, the protocols used in SAS were designed with the assumption that distances would be relatively short and extending the distances too far could result in degraded performance.

Number of Devices Connected
Another difference is the maximum number of devices that can be connected in a single domain. For FC, the maximum number of devices is 16 million, while for an IP-based system it is billions. SAS has a maximum number of over 16,000 devices in a domain. While this may seem smaller than FC’s 16 million, when FC is used as a device-level interconnect for disk drives it is in the form of an arbitrated loop, which has a limit of 126 devices per loop (or loop switch). While the connectivity and distance capabilities of SAS are less then what we would traditionally find in a fabric, today’s high-performance processors, high-density systems and storage design options enable configurations that can be constructed to meet many of the application needs currently addressed by traditional fabrics. Such new SAS fabrics have much smaller footprints and power requirements at very attractive cost/performance points, while maintaining the benefits inherent in the use of fabrics.

Cluster Environments
Below is an example of a cluster environment where the servers are either blades or in 1U rack-mount packaging. With these physical implementations, a large number of servers can be aggregated in a very small footprint. This configuration utilizes SAS to connect the servers to storage subsystems for both primary production level storage, as well as cost-effective storage for disk-to-disk backup and storage of reference data.

In addition, the configuration provides for attachment of tape storage for offsite backup and archival storage capabilities. Through the use of the SCSI extended copy command, the migration of data between levels of storage can take place without the need for data to pass directly through the server, reducing overall infrastructure bandwidth requirements. These capabilities have previously only been available with traditional fabric-level implementations of systems. SAS enables this to be done with a direct-connect architectural model, greatly simplifying implementation and management.


Figure 1: Clustered Storage Environment
Source: Maxtor


In this configuration, storage can be allocated to each server on an as needed basis, based on the applications being run on that server. New functionality is being defined for SAS expanders to include zoning, which will restrict the view of each initiator to only those storage elements it is permitted to access.

So, coming back to the original question, “Is SAS is a fabric or not?” As shown above, the answer really depends on the needs of users and how they intend to apply the features of a SAS configuration in their environment.

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