On Thu, Apr 22, 2010 at 08:41:18PM +0800, Wengang Wang wrote: > I got no idea why the value is illegal. > > Checking the codes, I found it also must be larger or equal to block size and it > must be in powers of 2, 2^N. Should we add these two limitation to man page? Yes, this should be added to the man page. Can you submit a patch to add it?