SUBSCRIBE TO OUR NEWSLETTER
Stay informed! Sign up to get expert advice and insight delivered direct to your inbox
RAID Gets New Stripe
Bigger and less reliable disk drives require better protection: Enter RAID 6
April 21, 2005
3 Min Read
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:
Ciprico Inc. (Nasdaq: CPCI) announced a 4-Gbit/s RAID 6 system for digital media this week at the NAB show in Las Vegas (see Ciprico Does RAID 6 for Digital Media). Digital media is considered a good candidate for early adopters of RAID 6 because SATA is popular in that market.
LSI Logic Corp. (NYSE: LSI) demonstrated its RAID 6 controller on an HP SAS system at the SNW tradeshow last week (see LSI Logic Demos RAID 6 SAS).
In early April, the Storage Networking Industry Association (SNIA) added support for RAID 6 to a draft of its Disk Data Format specs.
Hewlett-Packard Co. (NYSE: HPQ) in March said its upcoming SAS-SATA drives will support RAID 6.
Adaptec Inc. (Nasdaq: ADPT) in February said IBM Corp. (NYSE: IBM) will use its RAID 6 software with a SAS RAID controller expected to ship later this year (see IBM to Ship SAS RAID Server). The controller is part of a partnership with Adaptec and chip vendor Vitesse Semiconductor Corp. (Nasdaq: VTSS), which supplies the mixed signal and expander technology.
Intel Corp. (Nasdaq: INTC) introduced a storage processor for RAID 6 capabilities in February.
— Dave Raffo, Senior Editor, Byte and Switch
You May Also Like
More Insights
