RAID is nowhere near as reliable in practice as it appears on paper - one example is that you
frequently find, after a single disk (A) has failed, that another disk (B) 'fails during
recovery' - in reality, some sectors on disk B failed first but on a sector that did not get
read or written, so the bad sectors are only picked up during RAID recovery, which entails
reading the whole of disk B and others.
See http://www.nber.org/sys-admin/linux-nas-raid.html for coverage of some of these issues and
how NetApp and others would cover them. Personally I would only use ZFS, NetApp or something
similar that does a lot of media scrubbing, sector checksumming etc, in addition to basic