Cannot setup dumpdev on glabel disk

[Samuel Chow]

I am running 11-STABLE, and I am experiencing kernel panics when I am 
destroying a VIMAGE-based jail. Naturally, I flipped to the chapter 
about 'Kernel Debugging' to learn about 'Obtaining a Kernel Crash Dump'.

However, I am finding that my permanently glabel'ed disk partition 
cannot be used as dumpdev. Is that true, and why not? I mean, swap can 
use it just fine. I am unable to find this restriction in the documentation.


# grep swap /etc/fstab
/dev/label/boot01b      none         swap       sw      0 0
# swapinfo
Device          1K-blocks     Used    Avail Capacity
/dev/label/boot01b  41943040        0 41943040     0%
# glabel status | grep boot
   label/boot01     N/A  ada4s1
   label/boot02     N/A  ada5s1
# dumpon /dev/label/boot01b
dumpon: ioctl(DIOCSKERNELDUMP): Operation not supported by device