RAID 6, Take 2

New storage system controllers are taking their shots at trying to improve RAID 6 by eliminating performance hits that have slowed its adoption.

The AMCC 3ware 9650SE SATA II RAID controller family, which AMCC will sell through OEM partnerships, and Overland's Ultamus RAID SAN system both recently launched with the promise of improving RAID 6 performance. (See AMCC Ships Controllers and Overland Ships Raid System.)

It's too soon to know how successful they will be. AMCC's controllers won't show up in products for months. Overland says it has customers using its Ultamus systems, but one we spoke to says he hasn't tried RAID 6 yet.

Few question the need for improved RAID 6, though. "Originally, RAID 6 caused a big performance hit because of how they applied the algorithms," says analyst David Hill of the Mesabi Group. "I think these new controllers are kick-starting RAID 6."

The idea behind RAID 6 is to improve on RAID 5 by letting organizations recover from the loss of two drives simultaneously. However, RAID 6 controller performance typically drops dramatically when the second disk in the RAID group fails and two disks are being rebuilt. This has limited use of available RAID 6 systems, and made other vendors leery of bringing out RAID 6 controllers.The performance hit comes because it requires two sets of parity calculations to protect against two disk failures, and RAID 6 controllers ran the calculations one at a time. AMCC claims its RAID 6 implementations run the parity calculations simultaneously to try and avoid the performance degradation. Overland chief strategy officer Michael Kerman is more vague, saying Overland uses "special hardware to handle the extra calculations."

RAID 6 systems have been out there for a while; DataDirect Networks, for example, claims it shipped more than 20 Pbytes of RAID 6 systems over the last 10 months. (See DataDirect Ships Over 20 Pbytes.) Others including Adaptec, Promise Technology, and HighPoint Technologies have recently shipped RAID 6.

Dot HIll, Infortrend, SGI, and Xyratex say they have RAID 6 systems coming. (See RAID Rocks On.)

But there have been notable exceptions, especially among major vendors. EMC left RAID 6 support out of its product launch today, claiming it still comes at the expense of performance and capacity utilization. (See EMC in Full Launch Mode.) According to a statement released by the company, EMC considers other high-availability features, such as metaLUNs, media scanning, fault isolation, and better disaster recovery, more beneficial than RAID 6.

Not surprisingly, a few of EMC's major rivals disagree. Hitachi Data Systems offers RAID 6 as an option for its TagmaStore controllers, and Network Appliance offers a version of RAID 6, which it calls RAID-DP.HDS CTO Hu Yoshida says RAID 6 could be a lifeline for organizations using low-cost high capacity SATA drives. Yoshida says the real problem is RAID, and RAID 6 provides relief. Network Appliance also offers a version of RAID 6, which it calls RAID-DP.

"You need to improve the protection. RAID is becoming a problem. It's become old technology," Yoshida says. "People who experience drive failures know the value of RAID 6. When you do a rebuild, it takes the better part of a day. At least with RAID 6 we can defer the rebuild to another time and do a migration with our virtualization capabilities."

Others say that RAID 6 is sometimes worth the penalty hit, especially those using it on SATA drives for achiving and secondary storage where performance is not as crucial.

"Even a 10 percent penalty is well worth it for peace of mind," Mesabi's Hlll says. "If you lose a drive in a RAID 5 array you're on pins and needles until a rebuild takes place. If it fails, you have to go through the whole process of restoring from tape.

Another option is RAID 10, which lets users lose two drives without a hit in controller performance. It costs more than RAID 6 because it uses mirror sets and requires more disk, but buys even more peace of mind."I looked at RAID 6 but it didn't really offer me anything over the RAID 10 option," says Jay Vocaire, systems administrator at Cleveland-based health benefits firm Innovative Process Administration. "RAID 10 eats up more disk than RAID 5 or RAID 6, but [disk] storage is cheap now so that doesn't bother me."

As for Overland's Ultamus 5200 customer, Seattle-based Prowess Consulting IT director Nathan Johnson says he's using RAID 5 after using the system to set up his first SAN.

"RAID 6 is something we may implement down the road," he says. "In our scenario, we don't need RAID 6 technology."

Dave Raffo, Senior Editor, Byte and Switch