FreeBSD upgrade woes (8.3 -> 8.4)

[David Noel]

I have 4 servers I'm upgrading from 8.3 to 8.4. Two of them went
without a hitch, two of them blew up in my face. The only difference
between the two is the ones that worked have a 2-disk ZFS mirror and
the ones that didn't have a 4-disk ZFS striped mirror configuration
(RAID10). They both use the GPT.

After installworld && installkernel they made it through boot, but
right before the login prompt I'm getting a panic and stack dump. The
backtrace looks something like this (roughly):

0 kdb_backtrace
1 panic
2 trap_fatal
3 trap_pfault
4 trap
5 calltrap
6 vdev_mirror_child_select
7 vdev_mirror_io_start
8 zio_vdev_io_start
9 zio_execute
10 arc_read
11 dbuf_read
12 dbuf_findbp
13 dbuf_hold_impl
14 dbuf_hold
15 dnode_hold_impl
16 dmu_buf_hold
17 zap_lockdir

Does anyone have any idea what went wrong?

Does anyone have any suggestions on how to get past this?

Many thanks,

-David