RAID Rocks On

RAID (Redundant Array of Independent Disks) is nearly never in the news by itself these days, but the technology for protecting data on multiple drives is part of every SAN system announcement. According to industry sources, RAID is evolving to fit ongoing demand.

This week, for instance, EqualLogic's upgrade to its PS series IP SANs features the ability to manage multiple RAID levels as a single group. (See EqualLogic Firms Up Virtualization.) This kind of feature is cited by at least one analyst as part of the new look of RAID.

The ability to perform "RAID conversion, like from RAID5 to 0+1 with the data intact and in use and inside the array" is increasingly in vogue, says Greg Schulz, founder of the StorageIO consultancy

According to Schulz, other trends include the adoption of "RAID 6 or some variant involving dual parity to help protect against a double drive failure when using large capacity SATA and Fibre Channel disk drives." Also emerging are new kinds of RAID combinations, along with power management and optimization schemes.

At Detroit law firm Dickinson Wright, manager of operations Alan Wright cites features like these in describing his choice of EqualLogic's new gear. "If I want to use RAID 50, RAID 10, and RAID 5 in one array, I can manage them under one group instead of a different group for each RAID type," he says. (See EqualLogic Firms Up Virtualization.)Other suppliers also tout new streamlined RAID. EMC claims its Clariion systems can "create a LUN that spans multiple RAID groups" and can also "expand the size of the LUN dynamically when additional capacity is needed," according to spokeswoman Hadley Weinzierl.

DataDirect Networks says its S2A series of high-performance RAID systems allows users to mix and match RAID levels in various combinations. This feature is enabled by the ability of the system's controller to work at the bit level, says VP of product marketing Bob Woolery.

A word on the controller: In the world of RAID systems, claims about the controller abound, making it tough to discern technical from marketing arguments. A bit of probing reveals that while changing RAID levels may be pretty much transparent to the user, moving data from one volume to another in a RAID array is another story -- because the migration needs to be enabled by the controller, not the operating system.

Some vendors say this is purposeful. "We can move data from one volume to another and map the server to the new location, and this can be scripted to automate, but we feel there are issues in moving data without application-level awareness," writes LSI spokeswoman Diane Hughes in an email.

There is a trend to move the migration of data from the OS to the controller, and Xyratex is aiming to follow that eventually, says Tim Piper, director of product marketing for that vendor.More argument surrounds the implementation of RAID 6, a technique that adds parity striping to RAID 5, improving redundancy for double systems -- a kind of failover in case the failover doesn't work.

DataDirect Networks includes RAID 6 in its hardware, Dot Hill plans to offer it next year, Infortrend will ship it in selected systems this fall, while Xyratex has it in beta test. But LSI continues to evaluate it, and EMC and EqualLogic are turning a cold shoulder. "The perceived benefit of RAID 6 is marginal and comes at the cost of both performance and capacity utilization," states EMC's Weinzierl.

Bottom line? RAID continues to be a key to SAN success, as well as a source of confusion -- and flypaper for supplier claims. In the end, though, the fine points of RAID innovations will have to stand up to the realities of the data center.

One Infortrend customer puts it succinctly in a profile published on the vendor's Website. According to Jason Eicholtz, systems administrator for Medical Informatics Engineering: "We were looking for a combination of cost, performance, reliability, and build quality... When the host computer needs nothing special to use or see the storage array, that's a huge plus."

Mary Jander, Site Editor, Byte and Switch