Storage Basics: Choosing a RAID Controller

The last time we looked at server room components, we examined NAS and SANs. This week, let’s turn our focus to another storage corner of the data center — RAID controllers.

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.

Storage Options

Host-Based External Controller-Based Internal External All External 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.

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