RAID Products:
Entry to Mid Level:	StorageWorld Ultra2 RAID
	StorageWorld Fibre-U160 RAID | StorageWorld U160 SCSI RAID
	NAS RAID
Mid to Enterprise:	StorageWorld Fibre-Fibre SAN RAID

How to Specify Your Drive Array
First it is important for us clarify exactly what is meant by the term 'Drive Array'. At its simplest level, an array is basically a collection of hard drives physically connected together to form a large storage centre of data capacity. At this level, each hard drive exists in a number of states:

JBOD - Just a Bunch of Drives (usually pronounced 'Jaybod')

• The drives may share a common power supply.
• The drives may share a common data bus or backplane.
• To use the full total capacity of the hard drives each one must be formatted, mounted, and addressed individually.
• The sum total of the dives is not available as a single volume.
• Each drive is still independent of the other from the point of view of any operating system.

An array is therefore the basic building block of RAID data capacity. By building storage centres of suitable drive arrays, single logical volumes may be built by implementing RAID across the array offering potential storage capacity that is the sum capacity of the drives.

Physical Array Components - A standard array will consist of a number of components:

• The Array Enclosure: the cabinet itself in desktop, tower, or rackmount form factor
• Common Data Bus: usually built from a single backplane that extends through each drive bay
• Drive Bays: configured to accept slide in modules that lead directly to the backplane
• Drive Cannisters: the slide in modules containing the hard drive - usually fitted with a SCA interface for instant connection to the backplane
• The Host Interface: usually implemented in Fibre Channel or SCSI (LVD, HVD, SE, Ultra160) configurations
• A RAID Controller: implemented as an OS independent external hardware unit or a PCI-based control board and software combination

Many enclosures may also offer the additional option of extending the backplane to further extension array enclosures. For example, a Fibre Channel drive enclosure with a FC backplane may support further drive extension enclosures up to a maximum of 496 hard drives. This is possible because Fibre Channel supports multiple devices on a single data bus.

To combat interface restrictions a single RAID controller may have multiple drive interfaces. The selection of drive interface is especially important in environments where scalable storage requirements are of high priority. The Fibre Channel interface restricts the maximum number of nodes (devices) on a single bus to 126. This figure applies to both Point-to-Point and Arbitrated Loop environments.

Note: Enclosures that implement environmental control through the integration of SAF-TE may require a single SCSI ID for the exclusive use of the SAFTE bus.

Detailed specifications on the standards used in RAID array systems can be viewed at the RAID Advisory Board web site.


RAID Subsystems: Enclosures
When considering system enclosures for use as part of a RAID system there are a number of factors to bear in mind. The primary consideration is to decide whether you wish the enclosure to simply hold the hard drives independently to the controller, so that they may then be attached separately to your controller source of your choice, or opt for an integrated array system with controller and drives in a single cabinet. Both models have different advantages.

Integrated Controllers
The controller will usually come installed in a slide-in drive module similar to the hard drive modules. This allows the controller to be quickly exchanged in the event of failure. You may also be able to initially purchase the drive array enclosure without the RAID controller option and then add this at a later stage.

The disadvantages of this system are the total reliance upon the enclosure power supply, cooling system, and interface for continuous operation. If a critical component of the enclosure fails then the entire RAID system will immediately go off-line - even if all hard drives and controller(s) are operating normally and have not failed. The enclosure itself therefore becomes the single point of failure for the storage subsystem.

External Controllers
These eliminate the reliance upon enclosure integrity for supplying power, cooling, and host interfaces. External controllers are ideal in situations where more than one drive array is attached. If one enclosure fails, the other enclosures will still operate normally and data held on these enclosures will still be accessible.

The main disadvantages of this configuration is the necessity for standard external cabling between the controller and the drive enclosures, and the requirement for additional power supplies and cooling systems.

By separating the RAID processors and drive storage into separate elements, it is intended to make any array or RAID system you have, or may purchase, modular in design. By utilising industry standard interfaces and protocols it can allow you to upgrade existing systems even if they had not been specifically designed for this purpose originally. 

Obviously if you have an existing controller you may be wishing to upgrade or increase the drive capacity available to it, and you will require additional hard drive storage space only. Therefore you will not wish to purchase a new control system just to increase capacity. Your only need will be Drive-only Enclosures that supports your interface and capacity requirements. With most Drive-only enclosures now supporting industry standards such as Ultra-Wide SCSI, LVD SCSI (Ultra2, Ultra160 and above), and Fibre-Channel interfaces - with the addition of SAFTE and even communication ports built into some - adding additional hard drive enclosures is relatively painless.

If you are considering an entire new system including controller, then Controller-Drive enclosures may be a good place to start unless you have a specific Controller in mind that you wish to implement separately. Existing and predicted future capacity requirements also have a critical bearing on the system you choose.




RAID Subsystems: Specification Options

There is a maze of options for sorting through that you will need to drill down when it comes to specifying the exact RAID subsystem that will suit your requirements. Listed below is a brief summary of some of the options available to assist you in eliminating components and solutions that are not applicable. Remember, we are always available to assist you. Some RAID and Array enclosures also include the facility to add backup devices such as DAT or AIT drives directly into the same cabinet allowing faster backup performance. Systems that include this options have it listed together with details on the additional modules available.

In order to help you find the system you need, you need to specify a number of items. If you search through the available systems by any option listed or simply browse at your leisure through any range, you will find a diversity of enclosures, controllers, and systems. Some sorting options are already in place and working, but some are still under development. If you have any suggestions to help improve the navigation please let us know.

• Desktop 
• Tower 
• Rackmount

As stated above, the capacity of each system may vary, with desktop systems having the least growth potential. Rackmount & Tower systems can usually be scaled upwards by a larger amount, with Rackmount systems leading the maximum capacity tables.