Serial ATA: opening new markets for ATA RAID - Serial ATA

Computer Technology Review, Nov, 2002 by Mike Wentz

The enterprise data center has long been the domain of SCSI and Fibre Channel storage technology and components. Nearly two decades of technology has produced a solution set targeted to enterprise data centers with requirements for very highly available data.

ATA technology, on the other hand, has been the domain of the desktop and home markets. Attempts by various vendors to move ATA technology into the server and enterprise domain have not been successful due to real and perceived technical shortcomings of ATA drives and controllers.

A new technology is now emerging that has the potential to disrupt the long-standing polarity of these two technologies. This new technology, called Serial ATA (SATA), merges ATA disk drive technology with serial-cable technology. Seems pretty simple, so why is it being hailed as the basis for a revolution in the way we deploy storage? To answer that question, we have to look a bit more closely at the current storage technologies, and their strengths and weaknesses.

SATA Vs. SCSI

SCSI drives and controller technology was developed for server-class, 24x7, high-availability systems. The cable is long enough to support external storage, and the drives are built of heavy-duty components and tested rigorously in the factory. The SCSI feature set also incorporate a number of high-end features to improve performance and reliability in multi-user, multi-threaded servers that maximize the transfer of data between the host memory and the controller, and maximize the efficiency of the drive head across the media. The cable-interconnect schemes support up to 15 drives per SCSI bus, allowing the connection of large number of drives, making SCSI an excellent platform for large storage systems.

ATA technology, on the other hand, was developed and optimized for the desktop computer. Because desktop systems typically do not run 24 hours a day, 7 days a week, and generally are not subject to heavy usage, the components used to construct ATA drives are not as robust as SCSI. However, the cost sensitivity of the desktop market has resulted in engineering and manufacturing techniques optimized to provide a highly reliable product for the very simple reason that a drive failure could wipe out all the profit in the sale of a desktop system. The ATA protocol is much simpler than SCSI, haying been designed for single-user, single-threaded operating systems, and therefore doesn't have many of the features of SCSI. Finally, the ATA cable length is very short, only 18 inches, and supports only two drives per cable, perfect for a desktop, but inadequate for large storage systems.

RAID plays an important role in storage systems. Originally invented to provide a means of insuring data availability in the event of a drive failure, RAID has become the standard way to build storage subsystems for data centers where 24x7 data availability is a requirement. RAID creates a system where the data is redundant such that the loss of a single drive does not impact the availability of the data.

It would seem then, that a RAID system using ATA drives would be an obvious solution for high-capacity, inexpensive storage systems--the drives are cheap and RAID "hides" the drive failures--so who cares if the ATA drives are not quite as fast or reliable as SCSI? I can still get a lot of reliable storage for less money... why hasn't it caught on?

There are a few factors that have kept ATA RAID from seriously challenging SCSI: The 18-inch cable limitation of ATA makes it difficult to connect large storage banks of ATA drives; the limit of two drives per cable makes it hard to build large capacity storage subsystems, where most controllers can attain reasonable performance with a maximum of four drives; SCSI drives and controllers have features that lead to increased performance that ATA drives do not have; early attempts at ATA RAID were generally poorly implemented, causing those who tried it to have a negative experience, establishing a reputation for unreliability for ATA RAID.

Recently, the quality and feature set of ATA RAID offered by various vendors has improved over what was available just two years ago. Many ATA RAID vendors now offer a SCSI device driver model to obtain the benefits of improved host to controller data transfers, and the ATA RAID controller feature set has improved to offer features like Dynamic Sector Repair, Command Reordering, Command Read-back, and Media Scan to provide a RAID feature set that compares very favorably with SCSI RAID. These new controllers virtually eliminate the difference in the RAID feature set between SCSI and ATA.

The benefit is obvious: with a RAID system built using ATA drives and one of these new controllers, the file server thinks it's interfacing to a SCSI device, and derives all the benefits of SCSI RAID, but at a much lower cost.

SATA, as mentioned earlier, merges a serial signaling technology to ATA drives. This is important because it solves a number of problems affecting the use of ATA drives in large storage subsystems: The cable is small and flexible, allowing for easy routing and better system cooling; the cable is longer by up to 1 meter, opening up the possibility of connecting drives outside of the server; the cable uses low-voltage signaling, allowing higher bandwidth with inexpensive components, thereby preserving the price point of ATA drives and simultaneously providing the foundation for evolutionary increases in bandwidth over time; low voltage signaling also eliminates the requirement for 5VDC, enabling less-expensive power supplies, motherboards, and drives; serial signaling techniques are used, aggregating command, address, and data signals on only four wire pairs, significantly reducing high-speed signal integrity issues, allowing for much-higher bandwidth.