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Serial Attached SCSI - Delivering Flexibility to the Data Center

Editor's intro:- If you think you already know SAS because you know SATA and traditional SCSI then think again. Sometimes disruptive technologies wear an unassuming disguise. In fiction, Clark Kent, Frodo Baggins and Buffy Summers at first seem harmless, but we see them change into Superman, the Ring Bearer and the Slayer. SAS too comes cloaked in plain garb - with a physical layer which looks a lot like SATA. But like the Incredible Hulk there are muscles rippling under that shirt - and you would be wrong to dismiss SAS so lightly. There's a lot more inside this interface than it says on the box as this informative article reveals.

This classic article - published here in October 2004 - was one of the 1st user education articles about the new technology which we now call simply SAS. It was linked from LSI Logic's home in December 2004 ("Maxtor and LSI Logic discuss the industry's transition to SAS").

Serial Attached SCSI - Delivering Flexibility to the Data Center
Kevin Gray, business development manager, Server Products Group, Maxtor click to see profile of Maxtor
David So, product marketing manager, Storage Standard Products Division, LSI click to see profile of LSI Logic

As parallel storage technologies such as SCSI and ATA approach their performance limits, the storage industry has begun a transition to next-generation serial technologies that meet the bandwidth requirements of today's enterprise data centers. One of the crucial serial storage interfaces emerging is Serial Attached SCSI (SAS). Designed as the next step in the evolution of SCSI technology, SAS meets the performance and availability demanded by I/O-intensive, mission-critical, online applications.

While SCSI began moving towards SAS, ATA technology was also evolving towards its own serial version, Serial ATA (SATA). While originally designed as a desktop technology, today SATA drives are increasingly being deployed in enterprise applications, as the need increases for a secondary tier of storage that offers high capacity at a low cost-per-gigabyte, without requiring the performance and availability of SCSI or fibre channel disks.

As there is no single disk drive that meets all of today's storage and business requirements, storage administrators have to properly allocate their data to the appropriate drives. Doing so with multiple systems becomes complex, more expensive and space consuming. However, systems designed with SAS enable IT managers to mix and match lower-cost SATA drives in the same enclosure as high-performance SAS drives, meeting the demands of both price/performance and capacity/performance.

Compatible Yet Different

When initially designed, the SAS interface was developed to leverage a common electrical and physical connection with SATA, enabling the SAS backplane to accept both SAS and SATA drives in a single enclosure. With this capability, a SAS array can provide a tiered storage environment in a single system.

While SAS gives administrators the ability to designate storage to the different drive types in a single array, it is crucial that they first understand the characteristics and differences of each interface so they can assign applications to the appropriate drive.

SAS leverages the manageability and more than 20-year history of parallel SCSI, which today represents the majority of the enterprise disk market. However, SAS takes SCSI to the next level to meet tomorrow's demands for mission-critical, heavy-I/O applications, such as databases, Web servers and transaction processing. These applications demand the highest performance with around-the-clock availability.

While both SAS and SATA leverage a point-to-point topology to deliver increased performance over their parallel predecessors, SAS has the distinct advantage in this area. The current SAS standard provides for speeds of 3 Gb/s compared to 1.5 Gb/s for the current generation of SATA drives. Next year, SATA drives will also move to transfer rates of 3Gb/s. The next speed increase for SAS has already been defined as 6Gb/s and is expected to come to market in 2007. Although the data transfer rates for SAS and SATA will soon be the same, SAS will continue to maintain performance advantages over SATA due to several factors, including:
  • A full duplex architecture, which allows data and command transfers to take place in both directions simultaneously, effectively doubling throughput.
  • A number of suppliers will provide a broad offering of high-performance 15K and 10K RPM SAS disk drives. Capacity and performance offerings for SATA drives beyond 7,200 RPM are expected to be limited. (Editor's note - that projection may be correct for commercial hard drives but in July 2004 Adtron shipped a SATA solid state disk aimed at military applications which would substantially outperform 15K hard disks on IOPS.)
  • Wide port capabilities, which allow for multiple high-speed physical links to be combined into a single faster high-speed port, thereby aggregating the bandwidth of those physical links.
  • Dual porting and support for multiple initiators, which enables the design of dynamic load balancing systems.
SAS gains a performance advantage through its support of multiple initiators, or the ability to support I/O requests from more than one controller at a time. With dual ports and multiple initiator support, SAS RAID arrays can implement dynamic load balancing, allowing I/O requests to be evenly spread across multiple controllers, leveraging the full processing power of all of them. Without this capability, the I/O requests can become skewed, and overload one controller, while the others may not be at full capacity. SATA technology does not support this capability.

Another key characteristic that separates SAS from SATA is the command set. SAS leverages the SCSI command set which has been developed over the course of 20 years to deliver the performance and availability required by today's primary enterprise applications. SCSI drives feature sophisticated command queuing and reordering algorithms that allow drives to deliver an increased number of IOPs. In addition, the SCSI command set provides superior error handling and reporting over ATA drives, enabling more robust error detection and correction.

While SAS evolves to meet the performance and availability requirements of today's mission-critical information, SATA has emerged as the technology that meets the demands of secondary storage, which consists of "business important" information that needs to be online, but only accessed periodically, and can tolerate lower performance and availability.

Today data continues to grow and accumulate at extraordinary rates, being fueled by factors such as government regulations requiring companies' transactional and operational data be readily retrievable; and large quantities of reference data such as medical information. For such data, IT managers need high-capacity storage at a low cost, making midline storage an ideal solution.
the Problem with Write IOPS

the "play it again Sam" syndrome
Editor:- Flash SSD "random write IOPS" are now similar to "read IOPS" in many of the fastest SSDs.

So why are they such a poor predictor of application performance?

And why are users still buying RAM SSDs which cost 9x more than SLC? - even when the IOPS specs look similar.
the problem with flash SSD  write IOPS This article tells you why the specs got faster - but the applications didn't. And why competing SSDs with apparently identical benchmark results can perform completely differently. the article
Midline storage leverages the capacity and cost effectiveness of SATA drives to keep less frequently accessed data online. Midline applications do not require the performance or system availability needed by many of today's mission critical applications. By implementing midline storage, IT managers can keep less frequently accessed, non-mission critical data online cost effectively.

SATA is also driving new applications in the enterprise data center. Nearline storage leverages SATA drives to enhance backup and restore processes. With faster data transfer rates than tape and random access capability, SATA disks reduce the time required to perform backups and improve restore times. This is essential as today's backup windows continue to shrink. While SATA has driven this new application, tape still plays a role as an archival medium.

As the volume of data continues to grow and new applications emerge, IT managers need to find ways to store their data in a cost-effective manner. In order to do so, they need to take into consideration applications' specific requirements such as performance, availability, scalability, capacity and costs, and measure them against the different type of disk drives available to them. While IT managers still need to properly allocate their information based on application requirements, systems based on SAS will provide a new level of flexibility that will allow a single system to meet a wide range of requirements.

This new level of versatility will reduce the risk of purchase decisions by providing greater deployment flexibility. With the ability to house both SAS and SATA drives, SAS systems provide both the performance and availability required by enterprise class, I/O intensive primary applications, while enabling SATA drives to meet the needs of secondary storage, where cost-per-gigabyte is key. Ultimately, SAS systems reduce storage management complexity and improve investment protection.
Later articles on this subject:-
  • SAS SSDs directory - tracking the rise of the SAS SSD market - since before it began to the present day.

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