It's not used for tolerating disk errors (typically in a production context you have RAID for that, and failures tend to be for the entire disk). It's used to reduce storage requirements via striping. See the QFS paper (http://db.disi.unitn.eu/pages/VLDBProgram/pdf/industry/p808-...) for a good description of how this works. The basic idea is that with RS you can get 3x replication by splitting the data into 6 pieces stored on different servers, plus three parity blocks. This requires 1.5x storage rather than 3x while still tolerating the loss of any three machines of the nine.