Limitations of conventional RAID-5 on the ATA platform inhibit promise of ATA in the enterprise - Tape/Disk/Optical Storage

Computer Technology Review, June, 2003

The emergence of ATA drives as a serious alternative to enterprise storage holds the promise of significantly reducing storage acquisition costs. This is amplified by the advent of Serial ATA, which brings features like hot-pluggability, CRC for all communications (including data, commands and status), and thin flexible cabling to further decrease the gap between ATA and more expensive "server" class drive. However, in order to fully realize the advantages of the ATA platform for enterprise storage, new software technologies are required to guarantee the reliability and maximize the performance of the platform.

Specifically, RAID technologies currently used with SCSI and Fibre Channel storage implementations are ill-suited for use in the ATA arena. The pervasive use of write-back caching and the high cost of NVRAM-based board solutions negatively impacts the reliability and price advantages of the ATA platform, introducing the possibility of corruption and data loss and negating much of the cost benefit for the enterprise user. Similarly, the clear attractiveness of RAID Level 5 for large capacity storage is all but eliminated because existing methods for implementing low-cost RAID-5 systems have severe limitations in performance or reliability. On the ATA platform, this results in the undesirable flight to RAID-10 for most types of workloads and directly reduces the cost benefit of the platform.

In order for ATA-based storage to achieve its full potential in the enterprise, it is necessary to understand the limitations of today's hardware-assisted RAID solutions as these attempt, imperfectly, to address the unique characteristics of the ATA drive platform. Particular attention is placed on RAID Level 5, which is the most promising RAID type given its natural application to the larger capacity storage applications that will dominate networked ATA adoption.

ATA Characteristics

ATA disk drives emerged in the late 1980s, as desktop computers began their ascent into the mainstream of the IT universe. ATA is an acronym that stands for "AT Attachment," a reference to the IBM PC/AT that served as the de facto reference specification for the desktop since its introduction in the early 1980s. Though synonymous with IDE (Integrated Drive Electronics), the ATA designation is the subject of various ANSI specifications that have evolved the platform over time and is generic to the category.

Since their initial shipments in 1986, ATA drives have grown substantially in volume. Today, ATA drive shipments outnumber SCSI drive shipments by a factor of 6 to 1. And they outnumber Fibre Channel drive shipments by a factor of 10 to 1. Their volume differences are accounted for by the continuing centrality of ATA's role in the highest volume segment of the PC universe, the desktop computer. Because of their substantial volume advantages, they are subject to far more significant price competition than higher end drive platforms, and on average cost between 3 and 4 times less than SCSI or FC drives. The result has been an increased desire by IT end users to employ ATA drives in enterprise data settings as opposed to using them exclusively in desktop PC devices and workstations.

As engineered products, ATA magnetic disk drives harness the same basic technologies found in higher-end drives that employed different interfaces, most common of which are SCSI and Fibre Channel drives. They employ platters, actuators and a variety of micromotors. As such, ATA drives take advantage of the rapid advances in these component technologies that all disk drive manufacturers are continuously exploiting. Ranging from greater volumetric densities to enhancements in seek performance, ATA drives leverage that same basic technologies as SCSI and FC drives.

However, ATA drives do have significant differences from higher end drive platforms, and these differences must be addressed if the ATA platform is to emerge as a enterprise class storage platform. The first major difference is that ATA drives are subject to different sorting criteria than higher end platforms. Quality control is relaxed because of the relative tradeoff in profitability and defect rates. Instead of 1 percent component rejection levels as seen in SCSI drives, ATA drives are typically subject to a less demanding 5 percent rejection rate. The other differences between ATA and SCSI flow from their different end use targets. Because they are intended for desktop computers, ATA drives use different motors that generate less heat and ambient noise than SCSI. They are also slower than their SCSI counterparts from a RPM basis, given similar design goals to minimize desktop heat and noise but also to maintain SCSI performance advantages at similar capacity levels. That is, drive manufacturers frequentl y release similar capacity SCSI and ATA drives with higher RPMs available first in the SCSI device.

To compensate for decreased performance, ATA drive manufacturers have employed a variety of techniques to enhance the ATA platform. The most important of these techniques is called Write Back Caching. Write Back Caching involves the use of small memory chips contained in the drive electronics that buffer data transfers to the ATA disk. By using these memory modules, which are typically deployed in 2MB to 8MB configurations, the ATA drive can signal the completion of writes more quickly than if it had to Wait until that data was completely transferred to the disk media. However, even as write back caching provides a performance boost, it introduces a series of reliability concerns that contribute to the failure of the drive platform to achieve enterprise-class acceptance. These and other obstacles to reliability in the ATA drive platform will be discussed in detail later.