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

Computer Technology Review, Jan, 2000 by Kris Land

* Four drives for the original data

* Four drives for the mirrored data

* One parity-drive (equivalent) for each stack (Two total)

* One global hot spare (standby drive on which data can be rebuilt if a drive fails)

This architecture now requires a total of eleven disk drives (Fig 1). Thus, seven drives have been added to protect data on the four (original) drives. This configuration can recover from a failed drive in either stack. Even if all the drives in one stack failed, the remaining drives in the surviving stack would still provide access to critical data. However, in this case, only one drive failure in the remaining stack could be tolerated. Overall, if multiple drive failures occur within each stack, access to the database is lost. Barring a total stack failure, its maximum protection is against the failure of three drives, but in a limited fashion (maximum of two failures in any one stack).

Looking at the same example using eRAID to achieve equal protection against multiple drive failure (Fig 2), protection against three-drive failure is achieved at less cost and overhead:

* Requires only eight disk drives compared toll for traditional RAID

* Requires less administrative overhead

Hence, if these disk drives cost $1,000 each, the eRAD solution saves $3,000 while providing better insurance, since any three random drives can fail and the system will continue to properly function. Many databases rely strictly upon RAID 5 with striping and parity for protection against drive failure because RAID 1 solutions are so costly. However, RAID 5 supports continued operation only in the event of a single inoperable drive at any one moment. Losing two or more drives under RAID 5 brings operations quickly to a halt. For the cost of adding just one more drive, eRAID mitigates the risk of data loss by providing the means to sustain up to two drive failures.

LAND-5 eRAID, however, can support continuous operation even in the event several drives fail. Thus far, LAND-5 has successfully tested recovery when 50 percent of the disk drives fail. With eRAID, network administrators can manually assign the level of desired drive-failure protection. In short, eRAID allows the user the flexibility of selecting the level of drive-failure protection to fit specific needs.

The tangible cost of eRAID is that an additional parity drive equivalent is consumed for each incremental protection level. For instance, if a user desires to protect a 100-drive storage system against the possibility of two concurrent drive failures, then the equivalent of two disk drive capacities will be allocated for eRAID parity-related data. Thus, while users can still read from 100 drives, they can write to only 98 drives, reducing usable storage capacity by two percent. Hence, protection from (say) five concurrent drive failures reduces data storage capacity by only five percent. As any Information Technology Manager will testify, this is a small price to pay for dramatically enhanced storage reliability.