RAID, or Redundant Array of Independent Disks, is a technology for keeping data on several hard disks that work together as a single logical unit. The drives could be physical or logical i.e. in the latter case one single drive is divided into independent ones using virtualization software. Either way, identical data is stored on all of the drives and the main benefit of using such a setup is that in case a drive breaks down, the data will remain available on the remaining ones. Employing a RAID also improves the overall performance as the input and output operations will be spread among a couple of drives. There are several kinds of RAID dependant upon how many hard drives 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 drives - whether it's written in blocks on one drive after another or it is mirrored from one on the others. All of these factors show that the fault tolerance and the performance between the various RAID types may differ.