Regression on -current with mdconfig or dd

Hi,

I found a regression on current:
The problem is meet with dd but I'm not sure it's the root cause.

Here is how to reproduce the problem easily with 3 commands:

dd if=/dev/zero of=/tmp/SOURCE.img seek=100000 count=0
mdconfig -a -t vnode -f /tmp/SOURCE.img
dd conv=sparse if=/dev/md0 of=/tmp/DEST.img bs=64k

=> This last "dd" works great on 10.0-BETA3 and older but failed on
-current (r258675 and r259096) with the message:
dd: /dev/md0s1: Input/output error

Regards,

Olivier