RAID, or Redundant Array of Independent Disks, is a technology of keeping data on a number hard disk drives which operate together as one logical unit. The drives could be physical or logical i.e. in the second case one drive is split into separate ones through virtualization software. In either case, exactly the same data is saved on all drives and the key advantage of employing this kind of a setup is that in the event that a drive breaks down, the data will remain available on the remaining ones. Having a RAID also boosts the overall performance since the input and output operations will be spread among a couple of drives. There are several types of RAID dependant upon how many hard disks are used, whether writing is performed on all of the drives in real time or just on one, and how the data is synchronized between the hard drives - whether it is written in blocks on one drive after another or it is mirrored from one on the others. All of these factors indicate that the error tolerance and the performance between the various RAID types can vary.