Feeding on the proliferation of PC LANs, user interest in RAID — Redundant Arrays of Inexpensive Disks — is building rapidly.
“As the network applications become more critical to the company, you’ve got to take significant steps to make sure that when the network goes down it doesn’t take everything with it,” said Roy Wilsker, manager of end-user servers for Kendall Co., a health-care and adhesives company based in Mansfield, Mass.
Wilsker said one of those steps he is considering is installing RAID products in his network servers.
“It’s believed to be potentially a $5 billion market and there’s not a clear market leader, so everyone’s rushing in,” said Seth Traub, storage-market analyst for International Data Corp., a market-research company based in Framingham, Mass.
Recently unveiled new products include Micropolis Corp.’s Raidion disk array subsystems, AST Research Inc.’s array controllers for its server line and IBM’s AS/400 RAID array.
In order to sift through the profusion of recently released RAID products, users like Wilsker need to clearly understand the technology — which lets several disk drives work together to boost reliability and performance — observers said.
“Anybody who is buying anything that’s complex and doesn’t understand it is looking for trouble,” said Joe Molina, chairman of the RAID Advisory Board, which was created four months ago to help clear up some of the confusion.
The Advisory Board was in part the brainchild of Molina, who spent the last decade promoting the Small Computer System Interface (SCSI).
Tired of facing customers with unfamiliar technology, Molina said he left a SCSI marketing job 10 years ago to start Technology Forums. The Lino Lakes, Minn., firm is educating both vendors and users on data storage-related topics.
RAID is currently in a positionsimilar to that of SCSI 10 years ago, according to Molina, and Technology Forums serves as a facilitator for vendors who want to elevate RAID beyond buzzword status.
So far, 24 companies, including IBM, Digital Equipment Corp., NCR Corp. and Seagate Technologies Inc., have signed on as board members.
Closer to being an advocacy group than a standards-setting body, the RAID advisory board is trying to sort through the technical fine points that separate RAID products and develop guidelines to make the products more uniform.
For example, the group wants to encourage all disk drive makers to make their drives’ spindle-synchronization mechanism work the same way, Molina said. If they did, RAID developers wouldn’t have to accommodate different spindle-synchronization signals, a bit of re-engineering that can add to a RAID product’s price.
Until standards are set, RAID can mean different things depending on a particular vendor’s point of view. Most vendors look to an academic paper written by professors at the University of California at Berkeley in 1987 to develop their form of RAID.
In that paper, titled “A Case for Redundant Arrays of Inexpensive Disks,” the technology was grouped into several categories (see chart, Page 81). Although RAID categories are called levels, they are not hierarchical.
Simply put, a drive array ties disk drives together so they can share the task of storing data. Should one of the drives fail, other drives in the array are there to keep the data intact. The RAID products spread the data around differently, depending on what type — or level — of technology is employed.
Generally, RAID employs striping, which distributes data evenly across the disks, and mirroring, which makes duplicate copies of data on separate disks.
Each type of RAID has its own advantages and disadvantages. RAID 5, for example, can cause drives to perform slower than RAID levels 0 or 1 because it takes extra time to compute and write error-correction data. However, RAID 5 affords the high level of data protection that many users require for their network servers.
In some RAID configurations, the drives store data faster together than a single drive alone. So a grouping of less-expensive slower drives can offer greater throughput than a faster, more expensive drive. For example, in some mirrored arrays, the controller reads alternate clusters of files from each drive simultaneously, then pieces the information together and delivers it to the PC. Thus, reading time is cut significantly when two drives are linked through mirroring.
However, some vendors implement those RAID levels with slight differences; some support a given level in hardware, and others support a level in software.
Still others have developed their own type of RAID. For example, Storage Computer Corp., of Nashua, N.H., is now selling a patented hardware design it calls RAID 7 (see story, below). The subsystem is the first RAID architecture to implement a truly standards-based data storage system, according to company officials.
Storage Computer Corp.’s president says his company has created a superior RAID product by defying conventional wisdom.
Ted Goodlander isn’t shy about saying that the Nashua, N.H., firm’s RAID 7 storage subsystem doesn’t fit into the six Redundant Arrays of Inexpensive Disks categories followed by most disk-array vendors.
Indeed, Goodlander claimed that Storage Computer (which is known as StorComp) was working on the basic technology for the product long before the publication of the so-called Berkeley papers, an academic work on disk arrays written by three University of California computer-science researchers that is often cited as the foundation of RAID products.
“So many people took that paper and said it was the Holy Grail,” Goodlander said.
Unlike other varieties of RAID, in which the disk drives rotate in sync, StorComp’s RAID 7 subsystem has an asynchronous design, he said. RAID 7 moves the drive heads independently of each other to increase the number of reads and writes that the array controller can handle, Goodlander said.
StorComp’s RAID 7 also utilizes special algorithms that help prevent the controller’s data cache from becoming saturated. As a result, the company claims its RAID 7 subsystem transfers data two to four times faster than other RAID subsystems and still provides fault-tolerance for as much as 141G bytes of data. The only downside to this configuration, says Goodlander, is the fact that it cannot be self recovered in failure scenarios. Instead, a RAID data recovery expert such as Hard Disk Recovery Services must be used in order to rebuild the array.
The RAID 7 desktop Read more…