Serial ATA Technology

IT professionals are beginning to turn to an old, reliable and proven drive technology that is being packaged into new products aimed at addressing the cost/performance equation.
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High-performance disk drives-such as Fibre Channel (FC) or SCSI -- which are used in the storage array systems of video media servers -- are generally chosen for their high-reliability, high-availability and high-bandwidth functionality. Depending on the amount of storage in a server system, these drives and their associated controllers, switches, chassis, power supplies and so forth become a significant portion of the overall cost.

These components make up 30 to 80 percent of the total system cost, so balancing the proper amount of storage with the need for availability becomes an important system consideration.

For some applications, especially in those larger systems that rely entirely upon disk-based storage for their record and playout operations, having all the recorded media housed on expensive drives may not be necessary.

If the media stored on the drives doesn't require the high-availability, high-bandwidth or high-performance requirements of FC or SCSI, such as nearline storage, it might make better economic sense to employ an alternative drive technology. Furthermore, for less cost, you can increase the amount of accessible storage tremendously, without sacrificing too much in performance or reliability.

WHAT'S OLD IS NEW

In the data-centric world, where transactional storage and data files consume mass amounts of drive storage space, IT professionals are beginning to turn to an old, reliable and proven drive technology that is being packaged into new products aimed at addressing the cost/performance equation.

Advanced Technology Attachment (ATA) has been used for years; the ATA drives are a low-cost alternative to FC disk drives, and are well-suited to nearline storage supplemental to the online storage of today's high-availability/high-performance video media server storage architectures.

Originally developed for the personal computer market, parallel ATA, also known as Integrated Drive Electronics (IDE), has been deployed in both the PC, and in recent times, the UNIX environment for drive subsystems. These lower-cost solutions render some limitations in reliability and performance, yet have paved the road to more advanced implementations-notably the Serial Advanced Technology Attachment (SATA) interface. The serial interface in SATA is replacing the parallel approach of the IDE because of its speed and connectivity advantage.

The feature sets already present in FC and SCSI drive technologies, such as hot-plug capability and command queuing, are, or soon will be incorporated into SATA technology, making SATA more appropriate for data- and media-centric applications. SATA also opens the door to enterprise nearline storage by adding performance and scalability for backup and archiving as well as addressing the demands placed upon systems for streaming media and certain video editorial functionality at the desktop level.

Parallel ATA's evolution over the past 20 years, while keeping pace with increases in drive-space capacity and areal density, remains limited to the 40-pin physical cable structure that permits only two drives per bus or two drives per cable, and 133 MBps of throughput.

The new advantages of SATA include factors such as a point-to-point connection, whereby multiple ports can be aggregated into a single controller. Each individual drive is addressable through single cable connectivity from drive to controller, which in turn mitigates the data traffic waiting period inherent in the parallel ATA architecture. This advantage permits faster data streaming and easier communications between drives.

ON THE HORIZON

Today, serial ATA technology already delivers 150 MBps performance to each drive. Development is currently targeting 300 MBps by year-end, with up to 600 megabytes by mid-2007, according to the Serial ATA work group's plan. Additionally, the operating systems are legacy-proofed (no changes necessary from parallel ATA), and for error correction-previously performed on the data being transmitted back and forth-serial ATA integrates Cyclic Redundancy Checking (CRC) on the command-and-data packet level, which improves and enhances bus reliability.

One of the characteristics of FC drives, command queuing, will be supported in the future on Serial ATA (as SATA II). Command queuing is used to optimize the order of workloads in disk drives, improving load balancing and targeting throughput for entire arrays of drives when coupled into high-bandwidth applications.

Some believe that serial ATA drives would not normally be used as primary storage in mission-critical or midrange storage environments, but they certainly recognize the value in providing that bridge between permanent archive (tape-based) storage and online, readily accessible storage.

This places the uses for serial ATA in the nearline domain, where there are significant cost improvements when high transfer rates (such as those available in FC drives) are unnecessary. Nearline features where SATA drives make sense include lower-cost/high-capacity storage, only moderate activity (less than 50 percent duty cycles), and continuous 24/7 operation under a low workload situation.

SATA technology could be applied in nonmission-critical applications, where transfer rates are more important than seek times, as in rich-media-content handling; disaster recovery sites where a clone of data from the local site is streamed to a catch server for the "just-in-case" application; as a tape cache or secondary backup in the primary site for program content-that data which is held in time between tape archive and online media-server systems; and in hierarchical storage management (HSM), where data is first cached to the media-ingest server, then moved to secondary cache until the determination for actual usage is made.

The growing trend to disk-based content storage and reproduction needs to strike a balance between expensive, high-availability/high-reliability systems-and less costly, easily scalable interim storage systems. SATA technology is being applied in data center systems today, and the movement for broadcast- or media-centric applications is not far behind.

Already, there are vendors seeking the integration of dedicated SATA-based midlevel storage systems as an answer to the datatape-only based nearline storage system. Expect to see this technology edge its way into the content-server platforms as the next iteration of the media server technology evolves.