Last night my Tumbleweed system (20190520) performed the regular btrfs
maintainance.  I was working at that time, and noticed that the application I
used did not respond anymore.  Other stuff seemed to continue working (e.g.,
bringing an editor window in front).  So I thought its the application and
tried to terminate it, but nothing happened.  Short after that everything on
the screen became unresponsive, keyboard LEDS didn't react on state change etc.

I finally pressed the power button to restart the system (laptop, no reset).

However, the boot did not succeed.  It hang, systemd reporting its waiting for
four processes.  Two of those were 'flush journal to persistent memory' and
'save rfkill switch state'.  For some time the console display showed those
jobs in a loop, then it would also stop and become unresponsive.
This was consistent, I tried some three times or so.

So I finally booted from an USB rescue stick.  I could mount the FS without
problems.  Not knowing better, I ran a 'btrfs scrub' on it, which finished
after some 1-2 minutes without errors.  Not sure what to do next, I rebooted
from the hard disk - and the system came up clean.

By now I assume the crashes/hangs were only temporary(?) and the system might
have returned after longer waits, but I'm not absolutely sure.

However, looking at the logfile from the first hang I found some issue where
balancing failed with a 'no space left on device (although it has/had 5.6G free
when starting).  More alarming was this one:

May 27 00:02:10 woodstock.pitnet btrfs-balance.sh[17806]: Done, had to relocate
1 out of 84 chunks
May 27 00:02:10 woodstock.pitnet kernel: BTRFS info (device sda2): balance:
start -musage=10 -susage=10
May 27 00:02:10 woodstock.pitnet kernel: BTRFS info (device sda2): relocating
block group 398848950272 flags
system
May 27 00:02:10 woodstock.pitnet kernel: BTRFS info (device sda2): relocating
block group 398815395840 flags
metadata
May 27 00:02:10 woodstock.pitnet kernel: assertion failed: root, file:
fs/btrfs/relocation.c, line: 804
May 27 00:02:10 woodstock.pitnet kernel: ------------[ cut here ]------------
May 27 00:02:10 woodstock.pitnet kernel: kernel BUG at fs/btrfs/ctree.h:3518!

Unfortunately the log ends with this line, so no more info.

The full system log starting with the btrfs maintainance call is available at
http://paste.opensuse.org/24906643 (1 month lifetime - should I rather attach
it?)