What is a RAID Controller?
The RAID controller is best described as a device in which servers and storage intersect. The controller can be internal to the server, in which case it is a card or chip, or external, in which case it is an independent enclosure, such as a NAS (network-attached storage) . In either case, the RAID controller manages the physical storage units in a RAID system and delivers them to the server in logical units (e.g., six physical disks may be used to ensure that one drive stays correctly backed up, but the server sees only one drive).
While a RAID controller is almost never purchased separately from the RAID itself, the controller is a vital piece of the puzzle and therefore not as much a commodity purchase as the array.
A RAID (redundant array of independent disks) system is simply a collection of disk drives that employs two or more drives in combination for fault tolerance and performance. RAID drives vary in robustness from Level 0 (data striping without redundancy) to Level 5 (data striping at the byte level with stripe error correction information).
Lots of Options
Like the controllers that manage them, RAID devices are internal or external to the server, and it’s here where the waters grow murky. Enterprises have five routes they can explore when choosing a controller and a device. The first choice to be made, though, is whether to go with an internal or external solution.
In external controller-based storage, “the RAID controller and the disk technology are all outside of a host-based server, and normally housed in high availability enclosures,” says Gartner research vice president Roger Cox. The second type is “host-based storage,” also known as direct-attached storage (DAS), which breaks down further into two categories, also internal and external. In simplest terms, host-based storage can be internal or external, while non-host-based storage exists autonomously and does not require a host server.
The following table breaks out the basic storage options.
|SATA||SCSI||JBOD Enclosures||iSCSI||Fibre Channel|
According to Gartner data, host-based storage accounts for 34 percent of the overall market for external storage, with the remaining 66 percent going to “fabric-attached” (network) storage. Cox expects this share to grow from 66 percent to 77 percent by 2007.
“From a price perspective, the delta between SCSI and SATA drives is over 5x for a comparably sized drive,” states Barbara Murphy, vice president of marketing for Applied Micro Circuits Corp.’s storage business unit.
AMCC, which completed the purchase of RAID management tools vendor 3ware last month, claims 3ware’s recently unveiled Escalade 9000 SATA RAID controllers bring SATA speeds closer to enterprise level, at least for certain applications. “For applications that have a high degree of sequential I/O (disk backup, streaming video, image capture),” Murphy says, “the 3ware 9000 series controllers with SATA drives will outperform any other SCSI or SATA controller on the market.”
Frugality has its limits though. “We would not suggest that you put a Serial ATA disk in a situation where it’s a highly transaction-oriented, high-performance, critical application environment,” cautions Cox. However, “as people get more experienced with Serial ATA technology, you’re going to hear ‘Whoa, holy cow, you know, I can buy this stuff a lot cheaper.'”
This may be bad news for vendors, as SCSI and Fibre Channel drives carry a higher gross margin than do ATA drives, according to Cox, who adds that they “would have to sell a lot more units in order to keep their revenue up and, more importantly, to keep their gross margin up.”
Another player in the SATA market is Intel. In early 2003, Intel began selling a four-port SRCS14L controller. The product tightly integrates the RAID controller with the server architecture. “So instead of just saying, ‘Go and shop for the motherboard and for the RAID card that suits your needs, and then, it’s up to you to make sure that it all works together,’ we say, ‘If it’s on our list as a validated combination, then you can be very confident that it’s a robust solution,'” says Steve Fingerhut, Intel product line manager for volume products marketing.
The adoption of the speedier SATA II standard may bring another ball to SATA’s court. “SATA will [occupy] a stronger market position as SATA II features arrive,” says Luca Bert, director of product & programs management, RAID storage adapters for LSI Logic. “With [features] such as native command queuing, enclosure management, and port multiplier support, SATA II will be a viable solution for an enterprise-level server, especially in the cost sensitive segment.”
Page 3: External Host-Based and Controller-Based RAID
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The other type of host-based RAID isn’t really RAID at all. External host-based offerings are commonly described as “just a bunch of disks,” or JBOD enclosures. A JBOD system is not configured as a RAID setup, and therefore we have elected to not to cover any specific JBOD offerings for this piece. For those interested in learning more, the DF2000J from LSI Logic and theFS4100 Fibre to SATA JBOD from Adaptec are two JBOD products worthy of consideration.
Meanwhile, on the external controller-based front, iSCSI, a flexibly routed storage technology, is gaining momentum and garnering market share. Fibre Channel currently dominates this space, however. To date, Network Appliance is the lone player in the iSCSI RAID arena.
“The problem that iSCSI currently is encountering is that Fibre Channel’s not standing still,” Cox says. “They’re bringing the price differential down on Fibre Channel host adapters and switches to a point where iSCSI’s losing some of its value proposition in terms of cost.” Cox sees value in iSCSI despite its slower speed, as “to me the primary value proposition of iSCSI, until it gets to be 10 Gigabit, will be interoperability.”
One external controller-based product banking on this interoperability is Digi-Data’s STORM family of RAID controllers. STORM Digi-Data CEO Bill Tomeo pins STORM’s modular architecture as its main strength. “The STORM controllers house eight completely independent channels in one box,” Tomeo says. “Each 1U box has eight independent Fibre Channels on it, so what we’ve done is eliminate the catastrophic failures that can occur in a back plane.” Controllers typically have controller boards that share a common back plane, which presents a problem should a catastrophic failure occur, he explains.
SAN and NAS offerings also fall into the external controller-based category. For more information about SANs and NAS, check out last month’s article.
With five types of RAID controllers and countless vendors peddling their wares, what’s an enterprise to do? Bert sums up customer considerations best: Ultimately, “the choices of ATA/SATA, SCSI, or Fibre Channel solutions will depend on the desired trade off between performance, connectivity, reliability, scalability, capacity, and cost.”
Feature adapted from ServerWatch.