practice

Doi: 10.1145/1629175.1629194

Article development led by queue.acm.org

As hard-drive capacities continue to outpace
their throughput, the time has come for a new
level of RAID.
BY ADAm leVen THAl
Triple-Parity
RAiD and
Beyond

How MUCH LonGeR will current RAID techniques persevere? the RAID levels were codified in the late 1980s; double-parity RAID, known as RAID- 6, is the current standard for high-availability, space-efficient storage. the incredible growth of hard-drive capacities, however, could impose serious limitations on the reliability even of RAID- 6 systems. Recent trends in hard drives show that triple-parity RAID must soon become pervasive. In 2005, Scientific American reported on Kryder’s Law, 11 which predicts that hard-drive density will double annually. While the rate of doubling has not quite maintained that pace, it has been close.

Problematically for RAID, hard-disk throughput has failed to match that exponential rate of growth. today repairing a high-density disk drive in a RAID group can easily take more than four hours, and the problem is getting significantly more pronounced

as hard-drive capacities continue to outpace their throughput. As the time required for rebuilding a disk increases, so does the likelihood of data loss. The ability of hard-drive vendors to maintain reliability while pushing to higher capacities has already been called into question in these pages. 5 Perhaps even more ominously, in a few years, reconstruction will take so long as to effectively strip away a level of redundancy. What follows is an examination of RAID, the rate of capacity growth in the hard-drive industry, and the need for triple-parity RAID as a response to diminishing reliability.

The first systems that would come to be known as RAID were developed in the mid-1980s. David Patterson, Garth Gibson, and Randy Katz of the University of California, Berkeley, classified those systems into five distinct categories under the umbrella of RAID (redundant arrays of inexpensive disks). 9 In their 1988 paper, RAID played David to the Goliath of SLED (single large expensive disks). The two represented fundamentally different strategies for how to approach the future of computer storage. While SLED offered specialized performance and reliability—at a price—RAID sought to assemble reliable, high-performing storage from cheap parts, reflecting a broader trend in the computing industry. The economics of commodity components are unstoppable.

Patterson et al. were seemingly prescient in their conclusion: “With advantages in cost-performance, reliability, power consumption, and modular growth, we expect RAIDs to replace SLEDs in future I/O systems.” 9 However, their characterization of RAID as “a disk array made from personal computer disks” was a bit too specific and a bit too hopeful. While RAID is certainly used with those inexpensive, high-volume disks, RAID in its de facto incarnation today combines its algorithmic reliability and performance improvements with disks that are themselves often designed for performance and reliability, and therefore