On Wed, Nov 28, 2018 at 04:33:03PM +1100, Dave Chinner wrote:

One thing that is going to make this more complex at the XFS layer
is discontiguous buffers. They require multiple IOs (and therefore
bios) and so we are going to need to ensure that all the bios use
the same bi_rw_hint.

Well, in case of raid 1 the load balancing code might actually
map different bios to different initial legs.   What we really need
is to keep the 'index' or each bio.  One good way to archive that
is to just reuse the bio for the retry instead of allocating a new one.