RAID, or Redundant Array of Independent Disks, is a technology for keeping data on multiple hard drives that operate together as one 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. Either way, the same information is kept on all the drives and the key advantage of employing this kind of a setup is that in the event that a drive fails, the data shall still be available on the other ones. Employing a RAID also improves the overall performance as the input and output operations will be spread among a few drives. There are several kinds of RAID dependant upon how many hard drives are used, whether writing is carried out on all drives in real time or just on a single one, and how the info is synced between the drives - whether it's written in blocks on one drive after another or all of it is mirrored from one on the others. All these factors imply that the error tolerance and the performance between the various RAID types may differ.