RAID, or Redundant Array of Independent Disks, is a technology for saving data on a number hard disks which function together as a single logical unit. The drives could be physical or logical i.e. in the aforementioned case one drive is divided into independent ones through virtualization software. Either way, exactly the same info is kept on all the drives and the main advantage of using this type of a setup is that in case a drive fails, the data will still be available on the remaining ones. Having a RAID also improves the performance because 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 carried out on all the drives in real time or just on one, and how the info is synchronized between the hard drives - whether it is recorded in blocks on one drive after another or it is mirrored from one on the others. All these factors indicate that the error tolerance and the performance between the various RAID types may differ.