Insuring The Reliability Of Fibre Channel RAID Storage - Industry Trend or Event

Computer Technology Review, Jan, 2000 by Kris Land

Why would a system need protection against more than one drive failure at a time? Isn't the reliability of today's disk drives so high that the chances of a multiple drive failure are remote?

Disk drive manufacturers publish Mean Time Between Failure (MTBF) figures as high as 800,000 hours (91 years). Yet, as one examines these claims, disk drive manufacturers readily admit that such claims are unrealistic. In fact, the practical life of a disk drive is five to seven years of continuous use. Information Technology managers can painfully testify that disk drives fail with great frequency. That's why all companies place emphasis on storage backup and there is such a large market for tape systems.

It is clear that the likelihood of a drive failure increases as more drives are added to a disk RAID storage system. For example, a terabyte of RAID 5 storage consisting of fiftyeight 18GB disk drives can expect a drive to fail every 44 days! Moreover, when one drive fails, the statistical odds of a second drive failing increase dramatically and if two drives fail, the odds of a third failure jump again. In short, the more drives configured in a RAID storage system, the greater is its potential for suffering multiple drive failures.

Also, disk drives configured within a RAID storage system can be of different ages, including a mixture of new and older drives. This profile increases the odds of a multiple drive failure.

The consequences of a multiple-drive failure can be devastating. Typically, if more than one drive fails, or a service person accidentally removes the wrong drive when attempting to replace a failed drive, the entire RAID storage system is out of commission. Access to critical information is not possible until the RAID system is re-configured, tested, and a backup copy restored. Transactions and information written since the last backup may be lost forever.

Extensive research and development by LAND-S has resulted in a set of software and hardware algorithms that augments RAID storage by performing automatic, transparent recovery from multiple drive failures without interrupting ongoing operations. Called "eRAID," these patented algorithms allow users to select the degree of disk-loss insurance desired. Continued operations are possible even in the event of N1 drive failures. Moreover, because these algorithms have exceptionally fast computational speeds, storage transfer rate performance actually increases under eRAID while adding virtually unlimited data protection.

eRAID consists of a series of software matrix array formulas. It involves breakthrough algorithms for accomplishing XOR calculations (which are the basis of RAID 5). eRAID dramatically alters the reliability of RAID storage by circumventing previous limitations on the number of permissible drive failures. With eRAID, all but one drive can fail (assuming sufficient capacity) and users will still have access to critical information.

HOW DOES ERAID DIFFER FROM TRADITIONAL RAID?

Today, the ultimate protection for critical information is accomplished through RAID 1 (mirroring), overlaying RAID 5 (striping with parity), and then adding a global hot spare. For example, if user data consumes four disk drives, then reliability is improved by replicating this data on a second "stack" of four drives. Within each stack, however, losing just one drive would make the whole database useless. To further enhance reliability, each mirrored stack can be configured as an individual RAID 5 system. Since implementing parity requires an additional drive, user data and parity information are now striped across five drives within each stack. This provides protection against the loss of a single drive within each stack. So, from an original database that required just four drives, this RAID configuration has grown to include: