On Sat, 13 Jun 2015, Jean-Yves Migeon wrote:
I think that the slowness is I/O disk bound there: both disks were
used elsewhere first, so both have quite different block content.
Given that these disks are rather slow (5400 RPM ones), I suppose that
they dislike 64k read => compare => then 64k write on one, hence the
parity rebuild takes quite some time.
I'd check if the write cache is disabled on the 2nd disk. That
typically makes the writes much slower. You can use dkctl or atactl to
check the current cache status and change it.