From: Arjan van de Ven <arjanv@redhat.com>

Readahead of raid0 was suboptimal; it read only 1 stride ahead.  The
problem with this is that while it will keep all spindles busy, it will not
actually manage to make larger IO's, eg each disk would just do the chunk
size IO.  Doing at least 2 chunks is more than appropriate so that each
spindle will get a chance to merge IO's.

(Neil fixed raid6 and raid6 too)