by Harry Mason
This article is the first in
a two-part series exploring the future of Serial Attached SCSI. Read the second part here.
There are a lot of terms being used for the latest information technology architectures. Big data, cloud computing, data centers and storage-as-a-service are a few and they are all various forms of enterprise computing. Within these environments, numerous technologies are used to assist with implementing and managing mission critical data such as Hadoop (an open-source project that allows enterprises a new way to store and analyze data), virtualization, OpenStack, dedupe and RAID. For building and maintaining enterprise storage, Serial Attached SCSI (SAS), and its predecessor SCSI, have long served as the foundation by providing a serviceable slot architecture which contributes to high availability, high reliability and a high degree of serviceability and usability.
Due to the forward and backward compatible nature of the technology, SAS preserves the logical SCSI command set from which most enterprise middleware is derived. That’s particularly significant for CIOs and IT managers in terms of investment protection. It also helps to sustain a rich legacy of device types and data formats by providing an important element of continuity within the enterprise. It becomes especially important when realigning an information technology center to adapt to the dynamic needs of today’s emerging storage platforms. Despite the changing requirements, SAS maintains a flexible architecture that can expand and grow over time while continuing to protect the industry’s legacy storage investments.
Ratcheting up the enterprise attributes of SAS and advancing the technology roadmap to reach speeds of 12Gb/s is just one way of keeping the storage innovation treadmill alive. One of the primary objectives of 12Gb/s SAS is to preserve the existing SAS architecture and keep compatibility with legacy 3Gb/s SAS and 6Gb/s SAS usage models while maintaining that all-important backward compatibility and intermatability with existing SATA and SAS products as they are deployed in either cable or backplane environments. The connectivity improvements supported by 12Gb/s SAS are also particularly notable.
By doubling the SAS transfer rate from 6Gb/s SAS to 12Gb/s SAS, the industry improves the cost for the performance and bandwidth achieved for each watt consumed, making it one of the most efficient high performance storage interconnect solutions. In effect, it allows new deployments to reduce the number of connections required per Gb/s of bandwidth.
When operating at 12Gb/s, there are also new capabilities within the SAS standard to maximize link utilization. This allows storage users to preserve existing 3Gb/s and 6Gb/s SAS drives so they can operate across these 12Gb/s SAS links and maximize the utilization on that link, effectively doubling the number of drives that can be supported per SAS link. This is very important for architectures that demand a large amount of capacity scaling, and support rich near-line storage environments.
12Gb/s SAS is also an inflection point for the industry to adopt the Mini-SAS HD external connector. In Figure 1 below you can see what those connectors look like. MiniSAS HD allows systems to take full advantage of the 12Gb/s operating speeds, but also provides a smooth upgrade path to active copper connections (20 meters) and optical connections (100 meters). It is also beneficial for inter-rack connectivity solutions and for using SAS as a primary host-level interconnect. The net result of these connectivity options is improved capacity scaling, a higher level of density, simplified connector cabling options and market consistency.
The Mini-SAS HD connector also has the capability of providing cable management facilities so that the cables themselves can be queried by the software to detect whether the cable is present or not. This adds significantly to the enterprise attributes related to SAS. As SAS expands its reach within the data center, more cables mean a greater need to manage this evolving infrastructure. If the cable failure does occur, information about the cable exists within the connector and can be reported upstream through the software servicing those connections. This is a way of increasing the overall reliability, availability and serviceability attributes of SAS while going increasingly faster.
The Mini-SAS HD connector also allows these cable management facilities to support devices operating at speeds less than 12Gb/s SAS, bringing an additional degree of availability to 3Gb/s and 6Gb/s configurations. STA will be providing additional data on the availability of all these components over the next several months.
12Gb/s SAS also gives storage users the optional capability of doubling the amount of SAS bandwidth/performance per slot. This is accomplished by simply repackaging existing SAS connections as a MultiLink SAS slot. Providing additional SAS links on an upgraded connector and provisioning this new compatible connector with additional power, effectively allows for increased performance density, while overcoming the 9W power limit typical of 2.5-inch SAS drive slots.
MultiLink SAS is envisioned to be slot-compatible, not only with existing legacy connectors, but also with the expected emergence of a new category of multi-function connectors. Multi-function connectors are conceived as being able to support legacy SATA and SAS links, as well as PCIe, and are a recent development in the storage community. The goal of MultiLink SAS is to co-exist with the standards that evolve in the SFF (small-form factor) Committee, an independent storage industry standards group.
Just to touch on a point that is getting some attention in the marketplace today, the MultiLink SAS connector has been defined as the SFF-8630. It is designed to be a superset of the 12Gb/s SAS slot connector, the SFF-8680. The changes required of this connector are to incorporate two additional links and their associated sidebands. The finalization of that work is still in process.
To summarize the improvements storage users can expect with 12Gb/s SAS:
- Higher I/O performance rates, providing an advantage to low-latency storage devices like SSDs or hybrid drives or drives that are sufficiently buffered;
- Faster link speeds, and optionally, the capability to provide additional links which improves the overall SAS bandwidth moving forward;
- The ability to scale systems in terms of the number of drives and overall capacity;
- Gains in both in terms of bandwidth available as well as the density of external connectors, which provides for a doubling factor in terms of the number of drives that can scale from a single adapter card; and
Connectivity enhancements that support the increased performance and keeps SAS at the forefront of storage device interfaces – an interface you would have on an SSD, and HDD or a tape drive - but also as a system interconnect and a fabric that can be used to weave together complex and large scale storage subsystems.
12Gb/s SAS Technology
From a technology standpoint, 12Gb/s SAS will continue to use the 8b/10B coding schemes which have been popular for most serial interfaces. By keeping the coding scheme consistent, it reduces the complexity of the component design and lowers the risk of bringing that technology to market.
Because of the higher speeds inherent with 12Gb/s SAS, the standard now includes a training sequence that can tune both the transmitter and receiver characteristics. This provides a more reliable connection to the storage device that is attached within the system and is a unique capability for 12Gb/s SAS.
The 12Gb/s SAS standard will continue to preserve the 6 meter external passive copper connection while alternatively allowing for active connections to achieve the longer distances – active copper to 20 meters and active optical to 100 meters. STA has worked to maintain a motherboard slot usage model that allows for slot-based designs as well as cable designs, and will continue to preserve that model with 12Gb/s SAS. It is anticipated that for internal use within a server or within a storage cabinet, the existing Mini-SAS (SFF-8087) high volume, very popular cabling connector scheme, will be a reliable 12Gb/s connectivity solution internally and will enjoy a long life as the dominant in-cabinet cabling solution.
The backplane slot, the SFF-8680, is a backward and form-factor compatible slot which upgrades the existing SFF-8482 slot connector, to improve 12Gb/s signaling with better electrical characteristics. Legacy SATA and SAS drives will all be supported with this upgraded connector.