Article development led by
As hard-drive capacities continue to outpace
their throughput, the time has come for a new
level of RAID.
BY ADAm leVen THAl
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.
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.”
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