SAS Goes the Distance

Author: Jeremiah Tussey, PMC Senior Applications Engineer

Today’s challenging datacenter environments demand flexible switching and connection capabilities for storage scalability. SAS storage architectures allow for extensible configurations by providing incremental connectivity solutions: passive copper cables, active copper cables, and active optical cables. The fundamental basis driving these solutions is the necessity for a variety of cabling options; depending on a SAS topology’s needs for tiered interconnectivity, segregation, and failover capabilities. Figure 1 shows these different available options and how they might be used.

Figure 1

The entry-level SAS connectivity solution (shown in orange) is passive copper cables supporting up to 10 meters1 of distance connectivity for convenient Rack-to-Rack connections. For the industry, passive copper cables are the most commonly implemented cable in SAS infrastructures, allowing for cost efficiency while maintaining robust interconnections. A variety of passive cable options are readily available from multiple vendors, in sizes as short as 0.5m, up to 6m, and even up to 10m+ in specific implementations. This broad selection allows simple and short connections in initial topologies, expanding to longer reaches as the topology grows over time. Additionally, the more recently defined mini-SAS HD cables now have the capability to inform the system of certain characteristics of the specific cable. With this information, the system can make adjustments to high-speed signaling parameters as needed to support the varying cable lengths. This allows 12Gb/s SAS systems to be fully adaptable and reliable, without compromise.

Entering the mid-level SAS connectivity solution space are the active copper cables (shown in blue) for up to 25 meters for increased Datacenter connectivity. Active copper cables are the lesser known version of copper cables implemented in SAS infrastructures, allowing for increased distance while maintaining robust interconnections. A variety of active copper cable options are readily available, in lengths from 10m up to 25m. Active cables include powered components in the connectors which improve signal launch and reception at each end of the cable. This allows for the longer reaching connectivity that is required for growing infrastructures, specifically supporting rack-to-rack connections. Active cables also incorporate the capability to inform the system of the type of cable connected to provide power management capabilities that allows the system to pro-actively enable power to the connector when an active copper cable is identified as being attached.

The “Longest Reach” level of SAS connectivity is provided via the use of active optical connections (shown in yellow) for up to 100 meters and beyond. This level of interconnectivity enables flexible and expansive storage switching and networking for SAS solutions. Active optical cables are more commonly implemented in wide-reaching networking infrastructures. However, optical connections in SAS topologies bring the networking capability to SAS infrastructures, allowing for connections out of the normal range of copper cables defined for use in SAS architectures, while still maintaining robust interconnections within the bounds of the SAS specification.

A variety of active optical cable options can be obtained, either in standard sizes like 100m, or in custom sizes like 30m, 60m, or even 200m+ in specific implementations. As previously introduced, the mini-SAS HD cables can also support and identify active optical cables for providing connection information back to the system. Additionally, the QSFP form-factor cable has been defined for use with SAS infrastructures, using a readily available form-factor with which traditional network infrastructure specialists are currently deploying today. With either optical connectivity option, the same power management implementation used with active copper cables can also be applied generally for any SAS-defined interface connection. The information that can be accessed within the cable’s management memory area allows the system to also enable power to the connector when an inserted active optical cable is detected. Thus, optical connectivity further extends 12Gb/s SAS systems beyond just being fully adaptable and reliable.

In addition to the various connectivity solutions, SAS provides IT professionals and data center architects the capability to effectively configure storage area networks (as seen in figure 2), with cable lengths and management options to fit various switched storage architectures.

Figure 2

Providing connectivity that supports scaling as you grow allows SAS infrastructures to support Multi-Switch Fabrics and high availability architectures. In addition, extensive connectivity options and support for SAS Switching enables Centralized Storage Management, simplifying the end user experience and providing resource allocation, load balancing, and selective storage access via SAS Zoning features.

“SAS Goes the Distance” by providing flexible storage connectivity solutions for; entry-level, mid-range, and “Long- Reach” options enabling a variety of storage topologies required to meet the industry’s constantly evolving needs.

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