A Guide to Raid Arrays

One issue that plagues the world of technology is hardware failure. Unfortunately this is something that cannot be predicted or avoided. Although there are different forms of hardware failure, a disk drive failure can be considered by some to be the most detrimental type of failure. A failed disk drive can cause the loss of some or all of your data. By using a RAID array, you can add a level of redundancy to your drives that will help prevent the loss of data in the event of a disk drive failure.

What is RAID?

RAID stands for Redundant Array of Independent Drives and is typically referred to as a RAID array. A RAID array adds redundancy to your system by using multiple disk drives to simulate a single drive. Rather than having one physical drive, a RAID array is comprised of multiple disk drives, each referred to as a physical disk. The physical disks are combined and the server interprets them as one single drive, know as a logical disk. Data is then distributed to each disk in such a way that if one physical disk fails, the logical disk will still be intact.

A RAID array is important to have if you need to secure your data with the highest amount of uptime. In the event of a physical disk failing, the RAID array will keep the logical disk active so that your data will remain available to you. The RAID array also allows the failed physical disk to be replaced, while still keeping your data online.

Although a RAID array will protect you in the event of a hardware failure, it is highly recommended that you consistently backup your data. A RAID array will not protect you against changes made to your data or files being deleted.

There are a number of different RAID array configurations available. Below you will find details on the two most common RAID array configurations in use today, RAID 1 and RAID 5.

RAID 1

RAID 1 is also known as a mirrored pair and requires two physical disks. Each physical disk contains the same exact data. When new data is added, it is added to both physical disks. The server sees the two physical disks as one logical disk. If either physical disk fails in the array, the remaining physical disk will function as the logical disk until the failed disk is replaced.

Since both physical disks will contain the same amount of data, the total available space will be the size of the smaller physical disk in the array. For example, if two 60GB physical disks are used, the space of the logical disk will be 60 GB. If a 40GB physical disk and 60 GB physical disk are used, the total size of the logical disk will be 40 GB. It is strongly recommended that the two drives used are identical in make, model and size.

RAID 5

A RAID 5 array uses a minimum of three physical disks. Data is spread out across each of the physical disks along with a blueprint of how the data is organized. Together, the physical disks are seen by the system as one logical disk.

The amount of space available for data will be determined by the number of physical disks present and the size of each physical disk. If the physical disks differ in size, the smallest size physical disk will be used in the calculation. The amount of space available for data can be calculated using the following formulas:

Examples:

Using 3 - 30 GB disks in a RAID 5 array, the total space will be the following:

Using 5 - 30 GB disks in a RAID 5 array, the total space will be the following: