What is a RAID hard drive?

by techtiptom

Originally posted on WIBC.com on 01/17/2011

Q:  What is a RAID hard drive?

A: The RAID frenzy actually goes back to the 1980s when three researchers from the University of California Berkeley put together a case for “Redundant Arrays of Inexpensive Disks,” which brought to life the acronym of RAID.

Basically, in a RAID system, two or more inexpensive hard drives are put together to create one single large capacity storage device that also offers an enhanced performance. When the RAID system was first introduced, it was mainly used by large businesses that needed a large amount of data storage space. I mean, yes, it can protect you against data loss if one of the hard drives fail, but it’s not really meant to be used as a backup solution for the average user.

Now, RAID can actually be classified into two different categories: hardware RAID and software RAID. On the hardware side of things, the storage system created by the two hard drives is managed separately from the host computer, which then shows up as a single drive to the computer. The host computer doesn’t even really have to know that a RAID system is being used. As far as hardware goes, some of the RAID systems are put into place with a special hard drive controller card or the RAID functionality can be built right into the motherboard.

On the other hand, for the software side, the host computer handles everything. It’s what makes the RAID system look like one single hard drive. Unfortunately, this method does decrease the overall performance of the RAID system a little bit, because it functions by using some of the system’s memory and CPU cycles, which makes it completely dependent on the CPU.

With all of that said, there are six different levels of RAID:

1.) RAID 0 – This level requires a minimum of two disks. It does provide improved performance, but it doesn’t account for data repetition or failures. With this level, data striping is used so that the data is all split up onto the two disks. So, yes, this method does work at a very high level of performance, but if either of the drives fail, all of that data will be lost for good.

2.) RAID 1 – This level is a mirrored set of two disks in which the data is replicated to separate hard disks in real time. It’s done that way to ensure continuous availability, currency and accuracy. This level does provide fault tolerance in case any disk errors or failures occur.

3.) RAID 3 and 4 – These two levels are combined together in a set of three or more disks. One of those disks is mainly used for error checking, so if that particular one fails, the other two will be able to continue working without the error checking function. On the other hand, if one of the data drives itself fails, all of the RAID data will be lost.

4.) RAID 5 – This level is also a set of three or more disks, but for this method, the error checking disk is split up between all three drives. In that case, one single drive failure can be handled, but if more than one drive fails, all of the RAID data will be lost.

5.) RAID 0+1 – This is a hybrid form of RAID that some manufacturers have implemented to try and give the advantages of each of the two versions combined. Typically this can only be done on a system with a minimum of 4 hard drives. It then combines the methods of mirroring and striping to provide the performance and redundancy. The first set of drives will be active and have the data striped across them while the second set of drives will be a mirror of the data on the first two.

6.) RAID 10 or 1+0 – RAID 10 is effectively a similar version to RAID 0+1. Rather than striping data between the disk sets and then mirroring them, the first two drives in the set are a mirrored together. The second two drives form another set of disks that is are mirror of one another but store striped data with the first pair. This is a form of nested RAID setup. Drives 1 and 2 are a RAID 1 mirror and drives 3 and 4 are also a mirror. These two sets are then setup as stripped array.  Just like the RAID 0+1 setup, RAID 10 requires a minimum of four hard drives to function. Performance is pretty much the same but the data is a bit more protected than the RAID 0+1 setup.