History 2003: What’s Behind InoStor RAIDn?

InoStor, formerly Land-5 and a partly-owned (55%) subsidiary of Tandberg Data since last year, is offering licenses for a new type of RAID, its patented RAIDn, which it claims allows for the same level of security as on a RAID-5, but with fewer disks.

Since the invention of RAID and its different versions, 0, 1, 2, 3, 4, 5 by 3 students of Berkeley University in the much heralded paper A case for redundant arrays of inexpensive disks, published in December 1987, there have been few major technological inroads on the subject.

Only level 1 (mirroring) and 5 (striping with parity distributed on all disks) are widely used. One does come across the occasional RAID-3, while -2 and -4 have little appeal. A few evolutions have emerged with RAID 10+1 and 5+1 and various permutations such as RAID-6 and Storage Computer’s RAID-7, but nothing that profoundly alters the basic theory.

What’s so special then about RAIDn, conceived by Kris Land, the founder of Land-5, and one of the primary reasons for Tandberg’s investment? In RAID-5, for each write, the parity check is written only once on one of the disks (while the parities are distributed across all HDDs). Thus, in the event of a drive crash, it can be replaced and the data reconstituted from the parities on the other drives. If 2 drives crash, however, the situation is hopeless. It is possible, for greater security, to add hot spare drives, but that still doesn’t rule out some kind of incident during reconstruction of a failed drive.

A higher level solution involves mirroring the RAID-5, which yields RAID 5+1. This architecture, however, is HDD-intensive. For example, for a RAID 5+1 with usable capacity on 7 drives, you need 16 devices, 8 in RAID-5, plus the mirror, which requires as many devices.

With RAIDn, the parity check is written several times (while still distributed on all disks), which is the secret of the new technology. This time, for 7 drives of usable capacity, one need add only 3 drives to store 3 redundant parities, for a tolerance of 3 drive failures.

At first comparison, you save 6 drives, for cost savings of more than 40% in HDDs.

With RAIDn, the user selects the level of security desired: one extra drive to protect vs. a single drive failure (the classic RAID-5 structure), or 2 or more.

The new software algorithm, written in C code, to be implemented soon in the StorageCab disk array subsystem from Tandberg, is available on Linux and will also migrate to Windows. It could also be implemented on silicon for better performance.

The only question that remains is the reality of RAIDn’s performance, since it requires more read and write operations than RAID 5+1. According to Tandberg, both read and write performance are on a par with conventional RAID-5 and faster than the typical RAID 5+1.

This article is an abstract of news published on issue 183 on April 2003 from the former paper version of Computer Data Storage Newsletter.

Note: InoStor was acquired by Tandberg on March 2005.