RAID technology

Redundant array of independent disk (RAID) is the technology of grouping several physical drives in a computer into an array that you can define as one or more logical drives. Each logical drive appears to the operating system as a single drive. This grouping technique greatly enhances logical-drive capacity and performance beyond the physical limitations of a single physical drive.

When you group multiple physical drives into a logical drive, the ServeRAID controller can transfer data in parallel from the multiple drives in the array. This parallel transfer yields data-transfer rates that are many times higher than with nonarrayed drives. This increased speed makes the system better able to meet the throughput (the amount of data processed in a given amount of time) or productivity needs of the multiple-user network environment.

The ability to respond to multiple data requests provides not only an increase in throughput, but also a decrease in response time. The combination of parallel transfers and simultaneous responses to multiple requests enables disk arrays to provide a high level of performance in network environments.

STRIPE-UNIT SIZE

The granularity at which data is stored on one drive of the array before subsequent data is stored on the next drive of the array is called the stripe-unit size .

You can set the stripe-unit size to 8 KB, 16 KB, 32 KB, or 64 KB. You can maximize the performance of your ServeRAID controller by setting the stripe-unit size to a value that is close to the size of the system I/O requests. For example, performance in transaction-based environments, which typically involve large blocks of data, might be optimal when the stripe-unit size is set to 32 KB or 64 KB. However, performance in file and print environments, which typically involve multiple small blocks of data, might be optimal when the stripe-unit size is set to 8 KB or 16 KB.

RAID level-5

RAID level-5 stripes data and parity across all drives in the array.

RAID level-5 offers both data protection and increased throughput. When you assign RAID level-5 to an array, the capacity of the array is reduced by the capacity of one drive (for data-parity storage). RAID level-5 gives you higher capacity than RAID level-1, but RAID level-1 offers better performance.

RAID level-5 requires a minimum of three drives and, depending upon the level of firmware and thestripe-unit size, supports a maximum of 8 or 16 drives.

The following illustration is an example of a RAID level-5 logical drive.

RAID level-5 example

Start with four physical drives.

Create an array using three of the physical drives, leaving the fourth as a hot-spare drive.

Then create a logical drive within that array.

The data is striped across the drives, creating blocks.

Notice that the storage of the data parity (denoted by *) also is striped, and it shifts from drive to drive.

A parity block (*) contains a representation of the data from the other blocks in the same stripe. For example, the parity block in the first stripe contains data representation of blocks 1 and 2.

If a physical drive fails in the array, the data from the failed physical drive is reconstructed onto the hot-spare drive.

Advantages and disadvantages

RAID level-5 offers the following advantages and disadvantages.

Advantages	Disadvantages
· 100% data protection · Offers more physical drive storage capacity than RAID level-1 or level-1E	Lower performance than RAID level-1 and level-1E