Continuing Innovation with 6Gb/s SAS Advanced Connectivity

Author: Cameron Brett, Manager, Product Marketing, Enterprise and Storage Division, PMC-Sierra
With Contributions by Harry Mason, Director Industry Marketing, LSI Corp.

Not standing still, 6Gb/s continues to innovate with the SAS Advanced Connectivity Roadmap. The industry is trending toward large-scale SAS deployments with thousands of drives, multiple levels of SAS expanders, centralized SAS switches (all incorporating fail-over clustering), server virtualization and bladed processor complexes. This expansion is beginning to tax the basic SAS cabling and connector constructs that have been prevalent over the first two SAS generations.

The market is constantly demanding much more from the SAS interconnect to service increasingly more complex storage systems. The need to manage these large topologies becomes critical as does the greater need for substantially longer cabling distances.

The SAS community has responded to the following demands with the Advanced Connectivity Roadmap improvements:

Superior Connection Scheme (Better than Mini-SAS)

  • Twice as dense as Mini-SAS to support higher port-count densities
  • Electrically improved – less cross-talk, better signal-to-noise ratio, improved passive signaling
  • Support for active cabling (copper and optical)
    (Mini-SAS is capable of supporting active copper for 6Gb/s SAS only)
  • Support for cable plant management (unavailable with Mini-SAS)

Improved Distances (active copper to 20m, optical to 100m)

  • Enables more box-to-box, server-to-storage and rack-to-rack connections
  • Increased deployment options – more flexible installations and system configurations
  • Improved scalability – more spindles, more storage, more capacity

Managed Connectivity (Connection discovery and cable management)

  • Improved serviceability
  • Improved reliability
  • Lower total cost of ownership (TCO) – lowers the cost of error isolation and servicing

Converged Connectivity (Clear, consistent roadmap for broad market adoption)

  • Identical connection protocols for 6Gb/s (active and passive) and 12Gb/s (active and passive)
  • Consistent management across connection types
  • Consistent plan for port management and port scaling

Next Up: Mini-SAS active copper and QSFP managed optical

Convergence for an industry and technology is carefully managed and planned. 6Gb/s SAS advanced connectivity is no exception. In the short term, two interim cabling schemes will make an appearance in early 2010 and will be OEM proprietary configurations.

Mini-SAS – Active Copper
The first short term solution to the connectivity roadmap allows the existing Mini-SAS connector to support active copper signaling up to 20M in cable length. Using one of the existing ground pins, power provided to an active cable can support this doubling – or even tripling – of existing SAS cabling distances. Some next-gen large storage deployments requiring longer rack-to-rack distances may adopt this connection scheme, primarily in homogeneous (under a single company brand) storage deployments.

It is unlikely that this method will be used for optical connections and because this scheme requires one less ground pin, it will be limited to 6Gb/s SAS systems only. Additionally, this approach still relies on existing Mini-SAS mechanical keys for cable management, which is only complicated by these longer cable distances.

Quad Small-factor Pluggable (QSFP) Managed Optical
A similar class of customer that places more emphasis on cable plant management for large topologies may choose to deploy existing QSFP optical solutions before Mini-SAS HD. The expectation is that most of these early uses will be homogeneous systems and will become the first generation of systems based upon the SAS connectivity management approach (See Figure 1).


Figure 1: QSFP optical cables and assemblies support cable plant management.


Mini-SAS High Density (HD): The Best Managed Interconnect Solutions for Both Passive and Active Connections

The short-term Mini-SAS active copper and the QSFP optical approaches complement the existing SAS connectivity but fall well short of meeting long-term SAS goals. Fortunately, the Mini-SAS HD connector is fully capable with providing these advanced connectivity needs across multiple generations. This connection arrangement is implemented such that the cross-talk and signal-to-noise ratio is much improved over the existing Mini-SAS connector. Even SAS systems today, operating at 6Gb/s, find it challenging to meet their system-level signaling requirements across cables, backplanes and connectors. A Mini-SAS HD connector operating at 6Gb/s promises additional system margin for these challenging connectivity environments (See Figure 2).

Furthermore, the Mini-SAS HD connector supports greater port densities and allows more SAS ports across either edge of a PCIe-based host adapter. It supports substantially greater numbers of ports within or between server and storage racks.

Given the ability to support active copper and optical connections, distances of 20 and 100 meters respectively, are achievable. This greatly improves the SAS ability to service larger storage architectures, enabling more inter-rack connections and providing more flexibility in configuring large topologies. Since SAS can still be cascaded in these environments, large architectures could conceivably span several hundred meters and support many thousands of drives.

As Mini-SAS connections were being deployed, it became obvious that restrictive keying mechanisms were necessary to prevent cables from being plugged into the wrong connectors. Several keying plans were used to identify universal connections, upstream connections, downstream connections, etc. This growing number of keying mechanisms has the disadvantage of requiring multiple cable stock-keeping-units (SKUs), which adds a degree of complexity when servicing these systems.

As the demand for ever-increasing amounts of storage grows, the number of interconnections grows equally fast. Many of these large topologies are servicing the needs of high-end data centers and large enterprises where system robustness and serviceability are critical. Detecting and correcting cabling problems in these substantial deployments can be unwieldy without the appropriate management facilities.


Figure 2: Mini-SAS HD interconnect, shown in x1 and x4 configurations (x8 assembly not shown).


Mini-SAS High Density (HD) connectors are the complete package. They support passive copper, active copper and optical cabling schemes, complete with full-cable management facilities. This interconnectivity construct is especially useful in external cabling environments and is also available as internal connections. An important SAS roadmap note, it is designed to be extensible to 12Gb/s SAS.

Mini-SAS HD greatly improves this situation by providing management facilities within the connection plan, allowing for support under software control. Cables can be detected, queried and configured dynamically. This ability to detect the presence of a cable and to determine if a cable has failed or been removed, vastly improves the serviceability of large configurations. The precise identity of the cable used can be stored at the connector, which minimizes the confusion in replacement servicing (See Figure 2).

It is always difficult to migrate away from existing cables. In the interim, cables that adapt Mini-SAS HD connectors into existing Mini-SAS environments will be popular. In addition, there will be a number of Greenfield opportunities for SAS, given these high-end capabilities, accommodating legacy environments will not be a substantial concern.

Early Mini-SAS HD deployments will focus on external connections that are challenged by today’s Mini-SAS approach; however, there is an opportunity to become the preferred internal and external connector and to expand substantially over time. Given the fact that Mini-SAS HD is manageable, reduces cabling confusion, supports both active and passive cabling, and is extensible to 12Gb/s SAS, ensures its potential for growth and acceptance as the converged connectivity solution for future SAS deployments.


Figure 3: External and internal Mini-SAS HD configurations.


Notice 16 external SAS ports are supported on the bracket end of a PCIe low-profile card. These external SAS ports may support passive copper, active copper and/or optical connections. It’s important to note, this is twice the number of ports achievable with the Mini-SAS connector.

Figure 4 (below) is a single-page summary of the complete SAS Advanced Connectivity Roadmap. Notice that the connectivity roadmap has been decoupled from the various revisions in SAS link performance. It is expected that 6Gb/s SAS systems will embrace Mini-SAS HD due to the improved signaling characteristics, the ability to manage the cabling plant, and the ability to support substantially greater distances well before 12Gb/s SAS arrives.


Figure 4: SAS Advanced Connectivity Roadmap features the converged Mini-SAS HD connector and SAS Connectivity Management.


(Note: The use of QSFP for optical connections and the revised Mini-SAS connection for active copper support are recognized by STA as short-term options, especially in homogeneous storage deployments, but are not encouraged and have been omitted from the official roadmap.)

SAS Connectivity Management: Improving and Simplifying Connectors and Cabling

SAS Connectivity Management is an integral part of the current SAS connectivity roadmap, as it supports connection discovery and cable management, which is imperative in large storage installations. As the demand for storage continues unabated, connectivity becomes an increasingly significant part of the system solution. Having the ability to detect the presence or absence of cables and knowing if the connections are configured as active or passive, greatly improves the manageability and serviceability of these vast storage solutions. SAS Connectivity Management provides a consistent way of detecting the presence of passive and active copper and optical connections, eliminating the need for mechanical keys required to reduce cabling errors in Mini-SAS-connected systems.

Improved serviceability is the keystone of the new SAS Connectivity Management system, as it controls rapid fault isolation and eliminates multiple SKUs. Finally, it improves uptime, minimizes configuration errors and promotes service cost savings.

Connectivity Management also provides the means for assigning power to the Mini-SAS HD connector. Different implementations may require different levels of power depending upon the number of external ports and the supported cabling distances. This ability to control the power allowed to each connector brings a great deal of flexibility to the host controller design and is particularly significant for PCIe-based adapters which are power constrained.

SAS Advanced Connectivity: Unleashing the Full Potential of 6Gb/s SAS

SAS will continue to improve enterprise storage management with the Advanced Connectivity Roadmap, which offers a long-term plan based on the versatile Mini-SAS HD connector and SAS Connectivity Management.

These connectivity advancements, coupled with new ways of scaling – such as SAS switches – satisfy the needs of very large storage solutions. They deliver increased reliability and better serviceability in large-scale deployments, providing the necessary distance and configurability when scaling large data centers. SAS’ ability to grow incrementally and to scale as industry needs require, differentiates the SAS approach to servicing the enterprise.

Industry-hardened stability offered by SAS, as well as the new Mini-SAS HD connector as the long-term converged interconnect, provides an important element of investment protection across the storage landscape. SAS Connectivity Management ensures consistent software management, reducing system faults associated with cabling errors and providing a standard mechanism for rapidly identifying and correcting for cabling failures. As SAS grows in the enterprise, look for a continuous stream of storage innovations to improve reliability, management and performance.

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