Playing around with the FastTrak SX4000 was a major learning experience. I already had an understanding of RAID and how it works, but there was still some facts I was not aware of. To make it easier for our readers, I've decided to post some definitions supplied by Promise Tech.

Introduction to RAID
RAID (Redundant Array of Independent Disks) allows multiple hard drives to be combined together to form one large logical drive or "array." As far as the operating system is concerned, the array represents a single storage device, and treats it as such. The RAID software and/or controller handles all of the individual drives on its own. The benefits of a RAID can include: higher data transfer rates for increased server performance, increased overall storage capacity for a single drive designation (such as, C, D, E, etc.), data redundancy/fault tolerance for ensuring continuous system operation in the event of a hard drive failure. Different types of arrays use different organizational models and have varying benefits. The following outline breaks down the properties for each type of RAID array:

Striping (RAID 0)
When a disk array is striped, the read and write blocks of data are interleaved between the sectors of multiple drives (see Figure 11). Performance is increased, since the workload is balanced between drives (or "members") that form the array. Identical drives are recommended for performance as well as data storage efficiency.

The disk array’s data capacity is equal to the number of drive members multiplied by the smallest array member’s capacity. For example, one 100 GB and three 120 GB drives will form a 400 GB (4 x 100 GB) disk array instead of 460 GB. The stripe block size value can be set logically from 16KB, 32KB, and 64KB. This selection will directly affect performance. Larger block sizes are better for random disk access (like email, POS, or web servers), while smaller sizes are better for sequential access.

Mirroring (RAID 1)
When a disk array is mirrored, identical data is written to a pair of drives, while reads are performed in parallel. The reads are performed using elevator seek and load balancing techniques where the workload is distributed in the most efficient manner. Whichever drive is not busy and is positioned closer to the data will be accessed first. With RAID 1, if one drive fails or has errors, the other mirrored drive continues to function. This is called Fault Tolerance. Moreover, if a spare drive is present, the spare drive will be used as the replacement drive and data will begin to be mirrored to it from the remaining good drive.

Spanning (JBOD)
A Spanning disk array (also aptly named as JBOD for "Just a Bunch of Drives") is equal to the sum of all drives when the drives used are of different capacities. Spanning stores data on to a drive until it is full, then proceeds to store files onto the next drive in the array. There are no additional performance or fault tolerance array features in this array. When any disk member fails, the failure affects the entire array.

Striping/Mirroring (RAID 0+1)
A combination of both above array types. It can increase performance by reading and writing data in parallel while protecting data with duplication. A minimum of four drives needs to be installed. With a four-drive disk array, two pairs of drives are striped. Each pair mirrors the data on the other pair of striped drives. The data capacity is similar to a standard Mirroring array with half of total capacity dedicated for redundancy.

Block And Parity Striping (RAID 5)
RAID 5 calculates parity in order to achieve redundancy rather than writing a second copy of the data, like RAID 1. Parity is distributed across the physical drives along with the data blocks. In each case, the parity data is stored on a different disk than its corresponding data block. RAID 5 makes efficient use of hard drives and is the most versatile RAID Level. It works well for file, database, application and web servers.

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