You got a linear target that consists of two disks. One drive (a)
supports barriers and the other one (b) doesn't. Device-mapper just
maps the requests to the appropriate disk. Now the following sequence
happens:
1. block x gets mapped to drive b
2. block y (with barrier) gets mapped to drive a
Since drive a supports barrier request we don't get -EOPNOTSUPP but
the request with block y might get written before block x since the
disk are independent. I guess the chances of this are quite low since
at some point a barrier request will also hit drive b but for the time
being it might be better to indicate -EOPNOTSUPP right from
device-mapper.