"zypper up" hangs while "Applying delta".  The hang is for hours / days (i.e.
infinite).

I've only observed this on VMs (running under kvm / qemu) running Leap.  The
VMs in question are all running ext4 for the root file system (not btrfs).

When this happens, 'ps auxwf' shows that several child processes are running.
  /usr/bin/python /usr/lib/zypp/plugins/commit/btrfs-defrag-plugin.py
  /usr/bin/python /usr/lib/zypp/plugins/commit/snapper.py

Killing the snapper process seems to allow things to continue (after
"applydeltarpm check failed."), but it will usually hang again several deltas
later.  Killing the defrag will usually allow it to continue again, but it will
usually hang again at some point, in which case I have to hit ctrl-c a couple
times to stop zypper entirely, and re-run it.