RAID, or Redundant Array of Independent Disks, is a technology of keeping data on multiple hard disk drives which operate together as a single logical unit. The drives can be physical or logical i.e. in the latter case one single drive is split into individual ones using virtualization software. In any case, the same information is saved on all the drives and the basic advantage of employing this type of a setup is that if a drive stops working, the data shall still be available on the other ones. Employing a RAID also improves the overall performance since the input and output operations will be spread among a few drives. There are several types of RAID dependant upon how many hard disks are used, whether writing is done on all the drives in real time or just on a single one, and how the data is synced 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 mean that the fault tolerance as well as the performance between the various RAID types may vary.