RAID, or Redundant Array of Independent Disks, is a technology of keeping data on a number hard drives which operate together as a single logical unit. The drives can be physical or logical i.e. in the second case one single drive is split into individual ones using virtualization software. In any case, exactly the same information is kept on all the drives and the main benefit of using this kind of a setup is that in the event that a drive stops working, the data will still be available on the remaining ones. Employing a RAID also improves the overall performance since the input and output operations will be spread among a couple of drives. There are several kinds of RAID based on how many hard drives are used, whether writing is carried out on all drives in real time or just on a single one, and how the information is synchronized between the drives - whether it is written in blocks on one drive after another or all of it is mirrored from one on the others. All these factors suggest that the error tolerance and the performance between the various RAID types may vary.