RAID, or Redundant Array of Independent Disks, is a technology of saving data on a number hard drives which operate together as one single logical unit. The drives can be physical or logical i.e. in the latter case one drive is split into separate ones through virtualization software. In either case, exactly the same info is saved on all of the drives and the basic benefit of using such a setup is that if a drive fails, the data will remain available on the remaining ones. Employing a RAID also improves the performance since the input and output operations will be spread among a number of drives. There are several types of RAID dependant upon how many hard disks are used, whether writing is carried out on all the 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 it is mirrored from one on the others. These factors mean that the fault tolerance and the performance between the various RAID types could differ.