RAID, or Redundant Array of Independent Disks, is a technology of keeping data on a number hard disks that work together as a single logical unit. The drives could be physical or logical i.e. in the aforementioned case a single drive is divided into separate ones via virtualization software. In either case, identical data is kept on all drives and the basic advantage of using such a setup is that in the event that a drive fails, the data will still be available on the remaining ones. Using a RAID also boosts the overall performance because the input and output operations will be spread among a few drives. There are several types of RAID dependant upon how many hard disks are used, whether writing is carried out on all the drives in real time or just on a single one, and how the data is synchronized 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 of these factors mean that the error tolerance as well as the performance between the various RAID types could differ.