InfoStor RAID 6 Information Sheet

Trends in RAID drives, interfaces, controllers

Users and integrators are evaluating RAID 6, SAS-SATA combos, and software-based RAID.
By Dave Simpson
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This year marks the 20 anniversary of the publication of the paper that launched the RAID revolution in the
storage industry. It's rare that a technology development in this industry truly qualifies as revolutionary, but
RAID fits the bill.
Over the years, there haven't been many radical departures from the fundamental concept of RAID, although
the original RAID configurations- or levels-have mushroomed well beyond the traditional RAID 0, 1, and 5 into
RAID 1E (aka striped mirroring, enhanced mirroring, or hybrid mirroring), 3, 6 (dual-parity RAID), and hybrid
configurations such as RAID 10, 50, 60, and others.
In the context of the popular Serial ATA (SATA) disk drives, RAID 6 has emerged as an important
configuration. This is due in part to the high capacity and perceived reliability drawbacks of SATA drives.
Virtually all RAID controller vendors have implemented RAID 6, albeit in different ways.
Although RAID 6 is typically described as the ability to protect against two simultaneous drive failures, that
description is somewhat fallacious because the likelihood of simultaneous drive failures-even with SATA-is
extremely unlikely. More accurately, RAID 6 protects against the failure of-or an error on-Drive B while failed
Drive A is being rebuilt. And since the higher the capacity of a disk drive, the longer the rebuild time, RAID-6
protection becomes most important in the case of SATA drives, which can store up to 1TB. Furthermore, the
larger the drive, the more potential errors.
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To date, the use of RAID 6 has not been widespread, largely because of the write penalty associated with it. But
most controller manufacturers claim to have minimized the RAID-6 write penalty. For example, Scott Cleland,
director of marketing at AMCC, claims that with AMCC's controllers there is only a 5% to 7% increase in the
write penalty when you move from RAID 5 to RAID 6, which is attributable to the company's simultaneous
parity calculations. In contrast, some RAID-6 implementations exact a 20% to 30% penalty on write operations.
However, it should be noted that in many of the applications in which SATA drives are used (e.g., nearline or
secondary storage, disk-based backup, etc.) a write penalty of even 30% may not be a significant drawback.
Cleland estimates that less than 5% of RAID users are taking advantage of RAID 6 today, but that "RAID 6
should become the new RAID 5 once users see the benefits, because you should be able to get virtually the
same performance as RAID 5 with double the data protection."