Serial ATA getting serious

Two years after the debut of Serial ATA, this technology has continued to excite as well as frustrate vendors adopting it. Here's one vendor's view and expectations for this technology's next iteration.

It was a rising star in data storage. The technology promised new data speeds for desktop and laptop PCs, pencil-thin cables and power-thrifty electrical designs that would spawn a generation of smaller, cooler-running systems, and a price point that would drive down the cost of storage in enterprise environments.

Designed as a replacement for ATA, the technology would be widely used in home and office PCs and stood to replace the venerable SCSI in an increasing number of workstations and entry-level servers. Eventually, some pundits predicted, it would pack enough performance and reliability to displace SCSI as the mainstay for enterprise computing.

Today, nearly two years after the first Serial ATA products came to market, the technology is delivering on many of its promises, leaving others unfulfilled and raising high expectations for a generation of capabilities to come.

State of origin
Parallel ATA has been the industry standard for connecting hard drives and other devices in desktop and mobile computers for more than a decade due to the technology’s connection simplicity, low implementation costs and growing performance. But increasingly complex board designs, growing signal-timing requirements and other technological challenges are preventing parallel ATA from keeping pace with rising desktop performance demands.

Enter Serial ATA, a technology designed chiefly for desktop systems as a replacement for ATA and to make smaller, faster computers possible. It was also billed as a low-cost storage interconnect for workstations and entry-level servers that would deliver greater performance and reliability than ATA. Ultimately, it would offer connectivity for external storage and enable fan-out devices for greater scalability.

Serial ATA also promised higher levels of scalability and reliability through serial signal transmissions. This method, unlike parallel ATA technology, would deliver full bandwidth to each storage device. The technology would also eliminate the need for master/slave jumpers on ATA devices to simplify installation.

When the Serial ATA blueprint was complete, the design called for a 10-year performance roadmap, compatibility with existing ATA software, a seamless transition path from ATA, and intent to target typical volume ATA applications. As a replacement for ATA, for example, Serial ATA would deliver more bandwidth to meet growing demand for higher performance gaming PCs.

Serial ATA added a hot-plug capability that would allow IT managers to expand storage on the fly and deepen ATA’s penetration into the workstation and entry-level server markets. However, this optional feature has not been widely implemented, so end users must first ensure both the Serial ATA controller and disk drive support hot-plug, and that the proper hot-plug cabling and backplane are used to avoid potential disk drive damage.

As a replacement for UDMA disk drives in desktop systems, Serial ATA has been a success. The initial promoters of the technology built strong and increasing industry support by maintaining a consistent and well-defined charter. The standards process was private, controlled by five companies, but well managed and open to broad industry participation. Today, Intel’s south-bridges include two Serial ATA ports and disk drives are available at multiple capacity and price points. Vendors are delivering on the promise of task file register software compatibility, bringing backward compatibility of Serial ATA controllers to existing ATA drivers already embedded in commercial operating systems.

More desktop platforms are standardising on Serial ATA as the primary disk storage interface and, at this juncture, a full transition from ATA to Serial ATA on the desktop is limited only by the availability of Serial ATA CD and DVD drives. Current CD-ROM and DVD drives – which use existing parallel ATA technology – remain in widespread use.

The original Serial ATA specification was never meant to meet the needs of the high-end server and networked storage markets. To control development costs, Serial ATA 1.0 was developed without enterprise-class capabilities such as an advanced storage management protocol, scalable bandwidth for large numbers of drives, broad command queuing, active dual-port capability, and support for simultaneous multi-initiator access required for clustered environments.

Desktop drives in enterprise environments
As Serial ATA moved from laboratory to production in preparation for desktop deployments, a new class of data was rapidly growing: reference information. More small and mid-size businesses with modest IT budgets were digitising information requiring bulk storage including e-mail, presentations, graphics and a variety of images including CAD/CAM drawings, medical X-rays and bank cheques, increasing demand for low-cost storage and forcing these businesses to seek a cost-effective alternative to high-end disk drives for storage of these infrequently accessed types of data.

At the same time, many companies began to assign a value to information based on age, classifying data into two types: primary and secondary. Primary data -- usually high-demand information used in applications such as credit card, bank and customer service databases, and e-commerce -- requires high-performance transactional disk drives that can withstand the rigours of round-the-clock access. As primary data ages, storage costs begin to outweigh performance, and businesses archive the data to lower cost disk drives in an Information Lifecycle Management (ILM) strategy that lends itself to using high-capacity Serial ATA desktop drives for bulk storage.

Near-line storage applications -- such as disk-to-disk backup -- are another example of a workload where capacity is more important than performance. Serial ATA drives are ideal for both of these cost-sensitive environments since Serial ATA storage is optimised for capacity whereas enterprise-class disk drives such as SCSI are optimised for performance and reliability. Disk-to-disk backup is a growing practice as more businesses use the Internet to support round-the-clock operations and need to stage data for backup on disk drives before moving it to tape storage to maintain uninterrupted business operations.

Desktop drives, because they are designed for the light duty cycle associated with normal business hours versus the 24x7 demands of enterprise storage, raise reliability concerns among IT managers wanting to deploy Serial ATA in the enterprise. High-port count Serial ATA RAID controllers have emerged to allay data availability fears associated with deploying numerous desktop drives in these mission-critical storage environments. RAID technology provides uninterrupted data access in the event of a disk drive failure. Looking ahead
With the completion of the Serial ATA 1.0 specification, the industry has turned to beefing up the protocol with enterprise enhancements. The goal is to provide a low-cost scalable interface that meets the infrastructure needs of high-performance transactional environments. Known as Serial ATA II Extensions, the updated specification defines features such as native command queuing, a 3-gigabit-per-second data transfer rate, basic enclosure services, staggered spin-up of disk drives, and new devices such as port multipliers to improve scalability and port selectors to improve reliability.

Some Serial ATA proponents predict that SATA II Extensions will ultimately replace the SCSI protocol, a highly unlikely outcome since SCSI’s robustness for enterprise applications comes from 20 years of industry investment. That work has produced interfaces that deliver the highest levels of system reliability, scalability, performance and manageability. All the major enterprise interface technologies – including iSCSI, Fibre Channel, parallel SCSI and Serial Attached SCSI – use the proven SCSI protocol.

Another barrier to Serial ATA’s use in I/O-intensive transactional environments is total cost of ownership. While Serial ATA disk drives carry a much lower purchase price than enterprise-class drives, the ongoing hardware and management costs from frequently replacing failed drives not designed to withstand transactional workloads overshadows any initial cost advantage. Serial ATA disk drives, for example, lack the mechanical heft and sophistication to support fast seek times, the random data patterns typical of transactional workloads, and the tolerance to rotational vibration caused when multiple drives in a single enclosure are seeking simultaneously.

SCSI is undergoing the same transition as ATA, moving from a parallel to a serial architecture, namely Serial Attached SCSI. In doing so, Serial Attached SCSI will generate a larger potential base of customers for Serial ATA storage since the Serial Attached SCSI interface will support both Serial Attached SCSI and Serial ATA disk drives. Serial Attached SCSI will maintain backward compatibility with existing Serial ATA connectors and cables, allowing IT managers to populate a single dual-mode controller or backplane with either drive type depending on the workload or application.

Serial Attached SCSI is the follow-on technology to Ultra320 SCSI and will ultimately be the primary disk interface used in enterprise-class servers. Serial Attached SCSI also offers a performance roadmap extending from 3 gigabits per second to 12 gigabits per second, the unique ability to aggregate ports for scalable bandwidth and external cabling for connecting JBODs (just a bunch of drives).

The Serial Attached SCSI 1.0 specification was completed and ratified by INCITS (InterNational Committee for Information Technology Standards) in December 2003. At the same time, the Serial ATA II Extensions 1.1 specification is nearing completion pending final changes and clarifications. The first Serial Attached SCSI plugfest to demonstrate broad Serial Attached SCSI interoperability among early products was held in March. Serial Attached SCSI products will be available by the end of 2004.

After launching in 2003, Serial ATA controllers are now shipping in volume, an increase driven largely by greater disk drive availability.

Serial Attached SCSI is changing the way IT managers and systems builders are thinking about deploying disk drives in enterprise environments. For the first time, a single backplane will support the deployment of both enterprise and desktop disk drives. As a replacement for SCSI, Serial Attached SCSI disk drives will serve workloads where reliability and transaction processing are chief concerns. Optimised for cost-per-gigabyte, Serial ATA disk drives address the growing need for low-cost reference information storage. By supporting both disk drive types, Serial Attached SCSI backplanes will be the primary interconnect infrastructure for both reference and transactional data in enterprise environments.

As a result, Serial ATA disk drives will find still a wider market once Serial Attached SCSI systems are available. In time, most Serial ATA disk drives in the enterprise will be connected to a Serial Attached SCSI infrastructure for two reasons: the Serial Attached SCSI interface offers unprecedented storage configuration flexibility and simplifies component design, testing, qualification, and inventory control while reducing related costs for system builders and IT managers.

Linus Wong is strategic marketing group director at Adaptec Storage Solutions.

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