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

One of the most significant developments in the ATA world has been the evolution of the platform from a parallel bus architecture to a serial one. This evolution was undertaken to accelerate the use of ATA in networked storage environments and it has proven to be a crucial step in raising the awareness of the platform in multi-drive configurations. Technically, the Serial ATA drive is a seven-wire replacement for the physical ribbon of parallel ATA with a variety of benefits for denser storage implementations. The most important of these includes the cabling change (which facilitates better airflow and easier assembly) as well as the addition of capabilities like hot-pluggability and a point-to-point topology that enables full data-path switching. The first Serial ATA specification was completed in 2000 and drives supporting serial ATA begin initial production runs in the second half of 2002. Major research houses like IDC predict that Serial ATA will dominate the ATA platform within three years, rising to a 95-99 percent share of new drive shipments by the mid-2000s. In the area of networked storage, IDC further predicts the possibility of Serial ATA commanding at least 20 percent of entry-level servers by 2004.

Serial ATA Characteristics

Several years ago, the ANSI Parallel ATA specification was amended with the Ultra DMA protocol, which brought advanced CRC algorithms into the ATA world. These have been carried into the Serial product. While this inclusion has addressed low-level data transfer integrity issues, a new series of problems have surfaced that stand to pose the largest obstacle to ATA acceptance in the enterprise storage world. These problems center around the use of RAID technologies that have been largely tailored and refined through their application to multi-drive SCSI and Fibre Channel storage. As ATA begins to enter the multi-drive network storage world, enterprising vendors are attempting to apply legacy RAID strategies to multi-drive ATA installations but are achieving mixed results. Today, all hardware-assisted RAID technologies native to the ATA platform--as well as ascendant software RAID packages--fail to address key performance and reliability concerns that are unique to the ATA market. By failing to address these pro blems, it is unlikely that the ATA platform will break beyond the entry-level category that IDC and others envision for it.

RAID-5

RAID-5 is one of the methods for achieving higher performance and greater resilience to drive component failure that was originally developed by the U.C. Berkeley RAID team in the late 1980s and early 1990s under the auspices of principal investigators David Patterson, Randy Katz and their students. RAID is an acronym that refers to Redundant Array of Inexpensive Disks, and the original RAID project was conceived as a way to exploit the benefits of high-volume magnetic disk drives by using strings of lower cost drives together, in order to achieve the same benefits as more expensive storage configurations popular in the high-end systems of the day. The groundbreaking work of the RAID team and the industry acceptance that shortly followed have made RAID strategies and resultant technologies the ascendant paradigm for dealing with magnetic disk storage today.