By Harry Mason and Cameron Brett, SCSI Trade Association
Serial Attached SCSI (SAS) has delivered a variety of benefits to end users and systems/storage integrators and OEMs, including increased performance, support for higher device counts (specifically, the ability to expand beyond direct-connect configurations and scale via expanders to hundreds of devices), and more efficient cabling. And with the increasing availability of 6Gb/s SAS hardware, performance has doubled.
This has led to accelerated SAS adoption in a variety of applications and environments – from direct-attached configurations with only a handful of drives to expanded configurations cascading multiple storage enclosures supporting hundreds of drives.
However, SAS has the potential to deliver more functionality to address broader market opportunities. As more and more data is transmitted and stored, the industry trend is toward large-scale SAS deployments with potentially thousands of drives, multiple levels of SAS expanders, centralized SAS switches, server virtualization and bladed processor complexes. This architectural scale-out requires that SAS cabling and connectors keep pace with enhanced SAS capabilities for more complex storage systems. As the complexity of topologies grows, the need for improved management support and longer cabling distances is also increasing.
Enhancements to cabling interconnects will benefit existing use cases while expanding SAS into new environments. The industry's response to these market demands is the SAS Advanced Connectivity Roadmap, which offers improved capabilities beyond today's Mini-SAS connector with the denser and more flexible Mini-SAS High Density (HD) interconnect. Mini-SAS HD offers a variety of enhancements in four main areas:
Improved connection scheme
- Twice as dense as Mini-SAS to support higher port counts
- Electrically improved for less cross-talk, and better signal-to-noise ratio and passive signaling
- Support for active copper and optical cabling
- Support for cable plant management
Improved cabling distances
- Active copper supporting up to 20m and optical up to 100m
- Enables more box-to-box, server-to-storage and rack-to-rack connections
- Increases deployment options – more flexible installations and system configurations
- Improves scalability – more spindles, storage, capacity
- Intelligent discovery and management
- Improves serviceability and reliability
- Lowers TCO with improved error isolation and servicing
- Identical connection schemes for 6Gb/s (active and passive) and 12Gb/s (active and passive) SAS
- Consistent management across connection types
- Consistent plan for port management and scalability
All of these Mini-SAS HD enhancements can be deployed in existing 6Gb/s SAS infrastructure. Users/integrators are not required to abandon existing cabling solutions; they can take advantage of these new capabilities incrementally and deploy them as needed.
Mini-SAS HD is designed with improved cross-talk and signal-to-noise ratio, and eliminates a challenge that 6Gb/s SAS systems face today with signaling requirements across cables, backplanes, and connectors. These improvements are also achieved while increasing port densities and allowing more SAS ports to be supported.
The ability of Mini-SAS HD to support active copper and optical connections increases the supported cabling distance up to 20 meters (copper) and 100 meters (optical). This significantly improves the ability of SAS to support 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.
The capability of SAS to support much larger topologies requires an improvement in cable management in order to increase the usability of such environments. As the volume of interconnects grows, system robustness and serviceability are critical. Detecting and correcting cabling problems in these environments can be complex without the appropriate management facilities.
SAS Connectivity Management
Improved manageability and serviceability are keystones of the new SAS Connectivity Management system. Enhancements aimed at improving uptime, minimizing cable configuration errors and promoting service cost savings deliver advantages to users/integrators regardless of deployment size, but significant benefits are possible in large-scale deployments with thousands of drives.
SAS Connectivity Management supports connection discovery and cable management under software control. Having the ability to detect the presence or absence of cables, and knowing if the connections are configured as active or passive connections, improves the manageability and serviceability of large deployments. Cables can be detected, queried and configured dynamically. The identity of the cable can be stored at the connector, which minimizes confusion about replacement servicing. SAS Connectivity Management provides a consistent way of detecting the presence of passive and active copper and optical connections, and eliminating the need for mechanical keys required to reduce cabling mistakes in Mini-SAS-connected systems.
Easing the transition
Despite the advantages of improved signal integrity, port density and manageability, it is always a challenge to migrate away from existing cabling schemes. Cable-mating designs that transform Mini-SAS HD into the existing Mini-SAS form factor will allow current generation customers to explore the advantages of Mini-SAS HD. Additionally, in the near term there will be a number of "greenfield" opportunities for SAS, given these high-end capabilities where legacy compatibility will not be a substantial concern.
Mini-SAS HD interconnects support passive copper, active copper and optical cabling arrangements, with full cable management facilities. This interconnection scheme can be particularly useful in external cabling environments. And it is designed to be extensible to 12Gb/s SAS.
The SAS Advanced Connectivity Roadmap has been decoupled from the various revisions in SAS link performance – a necessity because it is projected that 6Gb/s SAS systems will begin to incorporate Mini-SAS HD well before 12Gb/s SAS arrives.
SAS connectivity will continue to evolve, and by improving the signal integrity, port densities, cable lengths, serviceability and manageability, the Advanced Connectivity Roadmap will ensure the delivery of cost advantages and efficiencies. Coupled with new ways of scaling – such as SAS switches – SAS will be able to meet the needs of very large storage environments.
HARRY MASON is the president of the SCSI Trade Association (STA) and director of industry marketing at LSI, and CAMERON BRETT is the STA secretary and manager of product marketing at PMC-Sierra. For more information: www.scsita.org.
Interim enhancements for Mini-SAS
The vast majority of enhancements on the SAS Advanced Connectivity Roadmap will require Mini-SAS HD connectors. However, a few interim enhancements for existing Mini-SAS connectors are possible.
For example, Mini-SAS connectors can support active copper connections, which will allow cable lengths to extend up to 20 meters. Using one of the existing ground pins, power provided to an active cable can support this doubling – or even tripling – of SAS cabling distances. This support for longer cable lengths is expected to benefit systems requiring longer rack-to-rack distances; however, it will be limited mostly to homogeneous (single vendor) storage deployments.
System designs based on utilizing active copper with Mini-SAS interconnects will not gain the full benefit of other features such as cable management. And because it requires one less ground pin, it is limited to 6Gb/s SAS systems only, and will not be extensible to next-generation SAS solutions.
In cases where cable plant management and distance is critical, builders of large topologies may choose to deploy existing Quad Small Form-factor Pluggable (QSFP) optical solutions. Again, these implementations will be limited in capabilities and will not offer all the advantages of Mini-SAS HD connectors.
The Mini-SAS active copper enhancements, as well as the QSFP optical solution, are not generally recommended due to their limited capabilities and the relatively short period of usefulness.