ZFS/zpool command blocks ... locking up all terminals

[O. Hartmann]

I have a faulty pool with an ambiguous label and I tried to resolve
that problem. ZFS is at the moment highly active copying data from
several volumes to another.

Operating system:

11.0-CURRENT FreeBSD 11.0-CURRENT #1 r259522: Tue Dec 17 19:02:10 CET
2013 amd64

In one terminal I exported the pool in question and tried to list it
via "zpool import". But the this command sequence locks up the terminal
for an hour up!

In another terminal I tried to issue to command "zpool status" to watch
the status of the pools (I have several). But this terminal ist alos
locked up right now!

What is wrong here? I had such an issue in 10.0-CURRENT as well. It
seems ZFS is locking everything up and can only be brought back by a
hard reset! What is going on? Why is zpool locking up in trying to
display a label-scrambled pool while the zpool status is then also
locked up, but latter is supposed to show the status of the other,
healthy pools? This reminds me of single-threaded tools which looks up
every operation consecutively issued after the blocking command.

How is this to be solved?

Oliver
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