RAID, or Redundant Array of Independent Disks, is a technology of keeping data on multiple hard disks that function together as a single logical unit. The drives could be physical or logical i.e. in the second case one drive is divided into independent ones through virtualization software. In any case, the very same info is saved on all drives and the basic advantage of employing such a setup is that if a drive breaks down, the data will still be available on the other ones. Employing a RAID also improves 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 performed on all drives in real time or just on a single one, and how the data is synced between the drives - whether it's written in blocks on one drive after another or it is mirrored from one on the others. These factors mean that the error tolerance as well as the performance between the different RAID types may vary.