RAID, or Redundant Array of Independent Disks, is a technology for saving data on multiple hard disk drives that function together as a single logical unit. The drives can be physical or logical i.e. in the latter case a single drive is split into separate ones through virtualization software. Either way, exactly the same information is stored on all the drives and the basic advantage of using this type of a setup is that in case a drive breaks down, the data will remain available on the remaining ones. Employing a RAID also enhances the performance since the input and output operations will be spread among a couple of drives. There are several kinds of RAID depending on how many drives are used, whether writing is done on all of the drives in real time or just on one, and how the information is synchronized between the hard drives - whether it is recorded in blocks on one drive after another or all of it is mirrored from one on the others. These factors imply that the fault tolerance and the performance between the various RAID types can differ.