savecore is not picking up dumpdev if the device is not also swap

[Jan Lentfer <Jan.Lentfer@xxxxxx>]

When playing around with swap cache I encountered the following problem:

swap was activated on the SSD, dumpdev as set in rc.conf remained on the
"old" disk-based swap device.
When panicing the system with debug.panic a dump would be written to the
dumpdev (disk). When rebooting savecore never found this dump, but it could
be manually restored from the dumpdev with savecore /var/crash
/dev/serno/9SF12T4Y.s2b

Here are some bits from my system, that might be helpful:
/etc/fstab:
/dev/serno/CVGB004100GX040GGN.s1a       none            swap    sw
    0       0
#/dev/serno/9SF12T4Y.s2b         none                    swap    sw
     0       0

/etc/rc.conf:
dumpdev="/dev/serno/9SF12T4Y.s2b"

What is strange, is that the sysctl kern.dumpdev is empty:
# sysctl -a |grep dumpdev
kern.dumpdev:

Even if I redo the dumpon settings, it remains empty:
atom# dumpon -v off
dumpon: crash dumps disabled
atom# dumpon -v /dev/serno/9SF12T4Y.s2b
dumpon: crash dumps to /dev/serno/9SF12T4Y.s2b (22, 0x30031)
atom# sysctl -a | grep dumpdev
kern.dumpdev:

It is also not related to using serno (which thought might be too long or
sth):
atom# dumpon -v /dev/ad6s2b
dumpon: crash dumps to /dev/ad6s2b (22, 0x30031)
atom# sysctl -a | grep dumpdev
kern.dumpdev:

So, I think what is happening is as follows: On a panic the dumpdev is
correctly determined and the dump is written to /dev/serno/9SF12T4Y.s2b.
When rebooting, savecore is not able to determine the dumpdev because
kern.dumpdev is empty and consequently can't find the core.
If I enable /dev/serno/9SF12T4Y.s2b again as swap in fstab, even after the
panic->reboot (the unsaved core still resides on the dev) and then just
reboot (no panic/core) savecore will find the core and save it when the
machine comes up again, because savecore goes through all the swaps in
fstab. So it is not a problem of the core, the device or savecore itself, I
think. But savecore not "seeing" the dumpdev.

Jan