RAID Gets New Stripe

A groundswell of support has bubbled up over the past few months for RAID 6, which offers superior protection over RAID 5. The move keeps pace with increased use of SATA and SAS drives, which demand better protection.

RAID 6 fixes a critical flaw in RAID 5 -- namely, that it cant recover if two disks in an array fail at the same time. RAID 6, also known as "dual-parity" RAID, can sustain two drive failures in an array without loss of data.

RAID 6 stripes data on a block level across arrays just as RAID 5 does. But RAID 6 uses a second set of parity codes for each data stripe. Because RAID 5 uses only one set of parity codes, it can only reconstruct data on one spare disk. If another disk fails at the same time, data is permanently lost. If a second disk fails with RAID 6, data can be reconstructed onto a second drive.

Industry sources peg the chances of a second drive failing without dual parity at about 5 percent, while the chance of a third drive failing is minuscule.

RAID 6 costs more to implement because it requires special controllers as well as more drives -- at least four. RAID 6 writes data more slowly than RAID 5 because of extra overhead, but usually performs random reads faster because it spreads data over more than one disk.Despite the increased cost, RAID 6 may be worth it, not only because SATA drives are less reliable than Fibre Channel, but also because the density of drives has increased along with capacity. So while the failure per inch of the drives remains the same, larger drives result in more failures. This is the case for Fibre Channel as well, although most of the early RAID 6 announcements center on SATA and upcoming SAS drives.

Following is a list of recent RAID 6 developments:

— Dave Raffo, Senior Editor, Byte and Switch