SAS Evolves with the Industry
As many of you are now well-aware, Serial Attached SCSI (SAS) has made significant strides over the past several years as the replacement for parallel SCSI which has been the storage interface of choice in the server market. In fact, SAS is now the dominant storage interface offered by server vendors worldwide, overtaking SCSI shipments in 2007. With its ability to support both SAS and Serial ATA (SATA) disk drives, SAS is also making headway as the disk drive interface of choice for external storage in both JBODs and external RAID subsystems. And slowly but surely, SAS is beginning to penetrate a segment that until now has primarily been the domain of Fibre Channel – the host connection of those same external storage systems that are already adopting SAS as the drive connection.
Coinciding with the increased interest in, and adoption of SAS, is the need to address emerging market trends such as virtualization, improved scalability, better storage manageability and more robust data integrity. As such, the SCSI Trade Association (STA) and ANSI T10 Technical Committee set out to define new features in the SAS-2 specification to better position 6Gb/s SAS for external storage connectivity.
New Expander Capabilities for Enterprise Scalability
SAS today supports addressability well beyond what was possible with parallel SCSI, with the ability to connect multiple servers and thousands of storage devices. Scalability at that level often requires that the storage devices and/or subsystems be consistently assigned, or zoned, to operate with multiple hosts in virtualized server deployments. This ability to assign various operating domains for both shared and separate pools of storage is accomplished through a capability referred to as SAS expander zoning.
The 3Gb/s SAS expander vendors had implemented zoning schemes that were, in some cases, vendor-unique and not necessarily compatible with the zoning schemes of other vendors. Emerging interface capabilities are often introduced into the market in this manner and when sufficiently supported, eventually make their way into the standard. Expander zoning was standardized in the SAS-2 technical specification and adopted as a 6Gb/s SAS expander requirement. This standardized zoning scheme improves SAS’ ability to effectively support more complex topologies across multiple expander vendors, while increasing the number of supported zones from 128 to 256.
Unfettered and often unforeseen growth in data center storage capacity requires a flexible storage architecture that is able to scale on demand. The need to rapidly discover and configure these larger, more complex topologies also becomes a concern. Since SAS was initially and primarily intended as a parallel SCSI replacement, it sometimes lacked an efficient means to initialize even modestly large disk-drive configurations. By shifting more of the SAS topology discovery process from the host controller to the expander, and by providing the added capability of flexible table-to-table routing, 6Gb/s SAS now dramatically reduces SAS messaging during topology discovery. By reducing the volume of messaging, the time it takes to initialize these systems is reduced. This improved efficiency gives system builders the functionality required to discover, initialize and scale with the ever-increasing capacity demands that drive tiered-storage solutions.
Higher Performance, Longer Cables
One of the key elements in the progression towards 6Gb/s SAS is the industry’s desire to preserve the distance and compatibility offered by 3Gb/s SAS environments, all while essentially doubling the data transfer rate. As SAS finds its way into enterprise storage, cable distances become more critical for data center scale-out. Decision Feedback Equalization (DFE) (and its associated training sequences) is the innovation in the SAS-2 specification that allows SAS to double its performance and to maintain its compatibility with first-generation SAS deployments. The introduction of DFE essentially allows SAS cabling of up to 10m at 6Gb/s transfer rates, an increase from the 6m cables that are supported in 3Gb/s SAS installations, thereby offering even greater flexibility in system deployments for data center professionals.
These new features also allow SAS to keep pace with the throughput being offered in PCI Express 2.0 servers. The improvement in bandwidth allows more disk drives to be added to these high-performance SAS links without the need for additional host controllers or ports. In some cases, this has the effect of freeing up PCI Express slots for other system expansion needs and reducing cable congestion, all of which are important system benefits.
Cable/Connector Consolidation
When 3Gb/s SAS first emerged, the most convenient external cabling solution was the InfiniBand-style connector. The industry eventually demanded denser cabling solutions, and the small form factor (SFF) mini-SAS connector was quickly adopted. To minimize cabling options and to improve the overall integration experience, 6Gb/s SAS eliminates the InfiniBand-style connection scheme, consolidating on the mini-SAS connector for both internal (SFF-8087) and external (SFF-8088) connectivity.
Enterprise-Level Data Integrity
It has long been recognized that the data protection schemes used in parallel SCSI were incomplete, since they did not protect the integrity of the command fields. SAS subsequently adopted the T10 Protection Information Model that allows both data and commands to be protected from the application layer, all the way from the host to the storage system to the disk drive. Commonly referred to as the Data Integrity Field (DIF), this feature was adopted in the SAS 1.1 technical specification. In practice, this scheme has already been implemented in many 4G Fibre Channel solutions and is likely to become a de facto requirement for 6Gb/s SAS solutions moving forward. Broad industry support is expected for what is rapidly becoming a critical enterprise requirement.
Action Item: SAS Continues its Penetration into External Storage. SAS is already firmly positioned as the preferred storage interface within the server market segment. With recent industry pressure to leverage these infrastructure components deeper into enterprise storage, SAS is evolving, often in subtle yet significant ways, to secure its future within these demanding markets. SAS’ flexibility allows for a storage infrastructure that effectively tiers high-availability and near-line storage solutions. As 6Gb/s SAS furthers its scale-out proposition with topology improvements, better storage management, and greater bandwidth, it extends its utility far beyond that of a parallel SCSI replacement technology. The ongoing innovations and investments in SAS technology will make it an important vehicle for delivering power-efficient, virtualized server and secure storage solutions that are being demanded in data centers worldwide.
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