What is RAID?

Simple enough question, i think.

I was looking at several HDD's today and nearly all of them say something about RAID, can someone explain to me what this is?

Thankyou.

 

RAID stands for Redundant Array of Inexpensive (later independent to remove the idea of RAId being inexpensive) Disks.

RAID0 needs at least 2 hard drives works by "striping" your data, as in it splits all data traffic in as many portions as you have hard drives and places one portion on each drive and the next portion of data on the next drive, etc. This theoretically speeds up access times as the disks can present their portion of the information at the same time. However, RAID0 is considered by some not to be a true RAID method as it provides no data redundancy, as if one drive fails then all the data is lost as the missing parts of data render the rest unusable. A RAID0 logical disk is the sum of the size of all the physical disks in the array. RAI0 cannot tolerate the loss of any drive, as stated above.

There are differing opinions as to whether or not RAID0 actually makes a noticeable difference in day to day computing speed. I have no position on this as I have never used a RAID system, but I would assume that there is a difference but it depends on the rest of the hardware in use.

As for other RAID types, RAID1 mirrors data between two or more hard drives, meaning that if one drive fails the other one will kick in seamlessly, you will get an error informing you of the dead drive, then you swap the drive out and the RAID software will "rebuild" the data from the old drive onto the new drive, meaning that a single hard drive failure will never loose you any data. A RAID1 logical disk is the size of one of the drives in the array, as all drives should be the same size. RAID1 can tolerate the loss of all but one drive.

RAID3 or RAID4 need at least 3 drives. They wok by striping the data cross at least 2 of the drives, as in RAID0, but another drive is used for parity information to enable the data to be rebuilt if one data drive fails, or if the parity drive fails the RAID software rebuilds the parity information. The size of a RAID3/4 logical drive is the sum of the size of all the data drives. This does not include the size of the parity drive. RAID3/4 can tolerate the loss of only one drive.

RAID5 needs at least 3 drives, as in RAID3/4, but instead of having a dedicated parity drive, parity is distributed across the drives. So if you have three drives, A, B, and C, the first section of data would be striped across drive A and B and the parity would be written to drive C. The for the next section the data would be striped across B and C and A would receive the parity. Then for the next section the data would be striped across C and A and B would receive the parity, etc. The size of a RAID5 logical drive is the sum of the size of the physical drives minus the space needed to store the parity information. A RAID5 array can only tolerate the loss of one drive.

RAID6 needs at least 4 hard drives. It works the same as RAID5 except the data is striped to 2 drives and the parity is written to 2 drives. RAID6 can thus tolerate the loss of 2 drives.

RAID1+0 Requires at least 2 drives. The data on the drives is mirrored as in RAID1, and is then striped as in RAID0. It is commonly implemented with 4 drives to take advantage of the speed benefit.

RAID0+1 requires at least 4 drives. The data on the drives is striped, as in RAID0, and is then mirrored, as in RAID1.