Serial attached SCSI hard drives: performance and flexibility for today's data explosion.

Whether for a server or for a storage enclosure, Serial Attached SCSI (SAS) hard drive options provide a scalable, flexible, better performing, and efficient solution to go beyond the inherent limits of parallel technology--just in time for today's data explosion.

Research, online activity, desire and need for data isn't just causing a data expansion, it's literally an explosion. In a recent study at the University of California Berkeley, the rate of storage performance is growing 30 percent year over year. The report states that all "physical media--print, film, magnetic and optical--and seen or heard in four information flows through electronic channels--telephone, radio and TV, and the Internet" will at some point be stored for retrieval. This holds true at the enterprise level with data collection spanning email, security, and financial transactions.

Parallel SCSI and Serial ATA technology limits

Parallel SCSI ran its course from Wide Ultra to Ultra 2 and so on until Ultra 320, the latest version which transmits data at 320 megabytes per second. The next generation, Ultra 640 at 640 megabytes per second, remains elusive with the industry unable to make the jump to the next generation of parallel SCSI. But because SCSI is very robust, reliable, easy to use and employ, the industry wanted to transfer these benefits forward. This led the industry to officially halt work on Ultra 640 in early 2003 in favor of serial attached SCSI.

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Another newly emerging technology is serial ATA (SATA). SATA is found on many desktop PCs and has started to make significant inroads into the entry-level server and nearline storage markets, offering high capacity at a relatively low cost. But SATA is also limited in its scalability, reliability, and performance.

In a nutshell, parallel SCSI technology evolution has been maximized, and SATA--though a significant improvement over parallel ATA--has its device limits.

SAS hits the scene

In taking a look at the available storage technologies, as well as the strengths of parallel SCSI, SAS creatively melds the best of each. SAS incorporates some high-end features from fiber channel such as native dual-port hard drives, multi-initiator support and full duplex communication; and the physical interface leveraged from SATA for better compatibility and investment protection; and combines them with the reliability and ease of use of traditional SCSI technology. New features make SAS much more scalable and flexible than the parallel technology it is replacing.

Performance

To increase hard drive performance, the key is to push the number of I/Os. There is a limit to how fast the drive can physically spin the discs and move the read/write heads. The simplest way to increase storage performance is to add more discs into a server (or storage enclosure) and spread the data across those disks, distributing the workload among multiple disks.

With smaller drives, more drives can be fit into a server or storage array. More drives translate into better I/O performance. Today with Ultra 320 hard drives, a typical dual processor server might have six hard drives in it. With SAS and the smaller form factor drives, there is room for additional hard drives--increasing performance, while still reducing power consumption and allowing for better airflow.

Solving data center woes

Data centers are constantly moving to denser and denser server and storage platforms. The size of the servers is shrinking. Everyone is trying to consolidate more into less space, or more into the same amount of space. Within servers, processors and chipsets are requiring more power to run. At the individual equipment level, more power is needed and more heat is generated. With each device generating more heat and with data center managers stacking more devices in the racks, cooling the data center becomes an issue. Today's data centers can push power requirements to as much as 300 watts (W) per square foot. A typical 42U cabinet with servers and storage requires 5 to 8 kilowatts (kW) of load, and today's high-density servers and blades can push this to 12 to 14 kW and even peak as high as 20kW. As data centers continue to consolidate, cable management becomes more of an issue.

Plug away for managing power and heat

One of the greatest benefits of the new SAS technology is that the physical connector on the hard drive is about one quarter the size of what it was for parallel technologies. With the connector no longer dictating size, SAS drives have been designed into a much smaller form factor.

The small form factor (SFF) SAS drives require about half the power of 15K SCSI drives and as a result generate less heat. Using SFF drives also provides additional space at the front of the server allowing more air to be drawn through the equipment for improved cooling.

A full rack of servers with small form factor hard drives typically reduces power consumption by 500 to 1000 watts as well as the heat that would normally be generated from those power levels. Less heat and better airflow contribute to improving the data center's thermal management.

Future proofing technology investments

By designing in compatibility with SATA drives at the connector level, investments in technology are protected. SAS architectures can communicate with the existing 1.5-Gb/s SATA drives as well as 3Gb/s SAS drives. The SAS roadmap also extends to 12-Gb/s and beyond. SAS further enhances the point-to-point serial topology by introducing expanders that act as simple switches, allowing a significant number of SAS or SATA disk drives to be connected to a single controller. Additionally, expanders enable SAS storage solutions comparable to fiber channel arbitrated loop with many of the performance characteristics of fiber channel switch implementations, but at a lower cost.

By virtue of its multi-protocol support, SAS enables IT to deploy a single storage infrastructure and then choose the most appropriate storage for their application. Storage can be one of the most expensive pieces of a server or storage system purchase. In the past, users had to make a decision on a single storage interface when buying a server, and often had only one choice--SCSI. However, not every application needs high performance, high reliability SAS disk storage. At the same time, users would not want to place business critical applications and data on low-cost SATA drives which offer significantly less reliability. With SAS and SATA compatibility, IT can now Implement both. Systems can now be deployed with enterprise SAS disks for performance and reliability with business critical applications, while using low-cost, high-capacity SATA disks for less demanding nearline or reference data and non-critical applications.

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The future of storage is now

SAS was developed to meet the changing demands of mainstream enterprise class storage system customers. SAS allows customers ultimate configuration, flexibility, and simplicity in their storage environments. By accommodating both low cost bulk storage (SATA) and the performance and reliability demands of mission critical applications (SCSI), SAS minimizes technology investments while maximizing IT user choice and ease of deployment.

SAS holds the promise of the future for enterprise users demanding superior choice, easier connectivity, and greater scalability as server and storage requirements continue to escalate.

Richard Scruggs is product marketing manager, Industry Standard Server Division for Hewlett Packard Company (Houston, TX)

www.hp.com