Get ready for a big rush of 6 Gb/s SAS gear in the near future. 6Gb/s SAS is the marketing name that the SCSI Trade Association (STA) has given to the industry standard based on the SAS-2 specification, the next generation of the SAS protocol beyond the current 3Gb/s SAS interface.
"One of the key benefits of SAS is its commonality with the Serial ATA (SATA) electrical and physical connection, offering customers flexibility in deployment," said David So, product marketing manager for the Storage Components Group at LSI Corp. of Milpitas, CA. "With enterprise storage requirements constantly changing and becoming more complex, factors such as larger capacity, greater density and scalability are more critical than ever. SAS was conceived to meet these demands while providing the highest performance."
He reports that 6Gb/s SAS defines a short list of features (as defined in the SAS-2 specification) that must be supported, including:
6Gb/s data transfer rates – This is a doubling of the data transfer rate of 3Gb/s SAS.
Self-configuring expanders – This feature allows the expanders, vs. the host controllers, to discover any topology changes. The benefit is system performance (allowing expanders to perform discovery in parallel) as well as an increase in device scalability.
Standardized zoning scheme – This ensures interoperability between vendors' products and will ultimately help further the adoption of SAS in external storage.
Decision Feedback Equalization (DFE) – DFE reduces inter-symbol interference and will allow for cables up to ~10m at 6Gb/s speeds. Currently, 3Gb/s SAS cables are limited to 6-8m in length.
Spread Spectrum Clocking – SSC spreads EMI emissions across a wider range of frequencies, in anticipation of tighter regulations from agencies such as the FCC.
SAS, of course, was designed as the next-generation storage interface to Ultra320 parallel SCSI, meaning that it was originally intended for Direct-Attached Storage (DAS) environments, whether internal or external storage. But the ability of SAS to support both SAS and Serial ATA disk drives has led to rapid adoption of SAS as the drive interface in external storage, enabling new topologies of tiered storage architectures - SAS drives for high-availability, transactional applications, and SATA disk drives for reference applications. This ultimately yields cost savings to enterprise data centers, being able to customize storage deployments based on need. SAS was also a primary driver for the adoption of smaller drives.
"3Gb/s SAS was the impetus for the introduction of 2.5" small form factor enterprise drives," said So. "The trend towards 2.5" HDDs will continue with 6Gb/s SAS, resulting in lower power consumption and less heat generation, among other benefits."
LSI and other SAS vendors have already introduced several SAS-to-SAS external RAID subsystems as well as SAS switches. Further enhancements in 6Gb/s SAS will make SAS even more attractive as a host interface for external storage moving forward, including self-discovering/configuring expanders, a standardized zoning scheme, and the removal of other SAS limitations.
6Gb/s SAS components have also been announced by several different storage vendors. Production availability of LSI's components (controller/ROC ICs, expander ICs, host bus adapters, MegaRAID adapters, etc.) is in-line with industry estimates, which is expected to be in 1H09. So notes that the industry's first 6Gb/s SAS plugfest, where multi-vendor interoperability testing is done, isn't planned until 4Q08.
"STA states that end-user products/solutions typically are not available until 12-18 months after the first plugfest," said So. "Therefore, we can expect 6Gb/s SAS-based server and external storage solutions in late 2009 or early 2010."
Meantime, LSI offers a broad portfolio of 3Gb/s SAS solutions today and is in active development and testing with initial 6Gb/s SAS products. LSI recently announced the industry's first OEM samples of 6Gb/s SAS expanders as well as being involved in the first end-to-end 6Gb/s SAS server interoperability demonstration along with Dell and Seagate.
"LSI will continue development of 6Gb/s SAS products, from controllers/ROC and expander ICs, to host bus adapters and MegaRAID adapters, to external storage systems, to mixes and bridges, to custom disk drive controllers," said So. "You can expect to see further product announcements from LSI in the coming months."
The adoption curve on 6 GB/s SAS is expected to be rapid. 3Gb/s SAS, after all, has already replaced Ultra320 SCSI as the storage interface of choice for DAS in servers. The external storage market (both JBODs and external RAID subsystems) is also adopting 3Gb/s SAS, especially as the drive (or back-end) interface. This is due in large part to SAS' ability to support both SAS and Serial ATA disk drives, offering users the choice of drive deployment based on need. 3Gb/s SAS is just now beginning to penetrate the host (or front-end) interface of external RAID subsystems in the lower price bands, too.
"6Gb/s SAS introduces a number of new features that make adoption of SAS as a host interface even more attractive and feasible, and we fully expect SAS to continue to grow as both the host and drive interfaces of external storage," concluded So.
This article was first published on EnterpriseITPlanet.com.
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