I tried to update my 10-STABLE machine (full ZFS including root)
from r268480 (built on Jul 10) to r268734 (built on Jul 16), but after
make installkernel and reboot into single user mode, I couldn't proceed
further because "zfs mount -a" crashed immediately with segfault.
Because only read-only root fs was available, I couldn't do any
debugging (/usr resides in a separate fs). Actually any zfs or
zpool commands seemed to crash immediately. Of course I couldn't
I guess this incompatibility had been introduced by recent several ZFS
related MFC/MFV between two builds. I believe old userland should be
usable with a newer kernel (or I cannot follow the standard install
procedure). If it is an expected behavior, it should be mentioned in
% uname -a
FreeBSD elvenbow.cc.kyushu-u.ac.jp 10.0-STABLE FreeBSD 10.0-STABLE #0 r268480: Thu Jul 10 15:25:15 JST 2014 email@example.com:/usr/obj/usr/src/sys/ELVENBOW amd64
% what /boot/kernel.new/kernel
FreeBSD 10.0-STABLE #0 r268734: Wed Jul 16 12:36:48 JST 2014
Fixed in r268786.
For your existing system, please try booting previous kernel, sorry for the inconvenience.
I confirmed that the problem was fixed.