dmevent can not open shared object file libdevmapper-event-lvm2snapshot.so leads to lvcreate/lvremove being slow

[Mikhail S Medvedev]

1) lsb_release -rd
Description: Ubuntu Xenial Xerus (development branch)
Release: 16.04

2) apt-cache policy lvm2
lvm2:
  Installed: 2.02.133-1ubuntu7
  Candidate: 2.02.133-1ubuntu7
  Version table:
 *** 2.02.133-1ubuntu7 500
        500 http://ports.ubuntu.com/ubuntu-ports xenial/main ppc64el Packages
        100 /var/lib/dpkg/status

3) Expected lvcreate/lvremove test to take less than a second
4) It took about 30 seconds instead

To reproduce
============

Use the script.

  export GROUP='group-test'
  export VOLUME='volume-test'
  export SNAPSHOT='_snapshot-test'
  export BACKING_FILE=/root/lvmbacking

  if ! vgs -o name | grep $GROUP; then
    dd if=/dev/zero of=$BACKING_FILE bs=1M count=2000
    losetup /dev/loop100 $BACKING_FILE
    pvcreate /dev/loop100
    vgcreate $GROUP /dev/loop100
  fi

  lvcreate -n $VOLUME $GROUP -L 1g
  lvcreate --name $SNAPSHOT --snapshot $GROUP/$VOLUME -L 1.00g
  lvremove -f $GROUP/$SNAPSHOT
  time lvremove -f $GROUP/$VOLUME

During lvcreate/lvremove, /var/log/syslog would show:
Mar 23 15:39:59 xenial-server-2016-01-18-ppc64el dmeventd[2753]: dmeventd libdevmapper-event-lvm2snapshot.so dlopen failed: libdevmapper-event-lvm2snapshot.so: cannot open shared object file: No such file or directory.

Workaround
==========

In /etc/lvm/lvm.conf, change snapshot_library to use absolute path:

  snapshot_library = "/lib/powerpc64le-linux-gnu/device-mapper/libdevmapper-event-lvm2snapshot.so"

The workaround is obviously would only work for the specific architecture. A
proper fix would be to find out why dmevent can not load the library, and fix
that.

Additional Notes
================

I have also tested this on Xenial x86 VM, and on baremetal install of Xenial.
Had the same problem on both.

[Launchpad Janitor (janitor)]

This bug was fixed in the package lvm2 - 2.02.133-1ubuntu10

---------------
lvm2 (2.02.133-1ubuntu10) xenial; urgency=medium

  * Cherry-pick change from lvm2 2.02.133-2 in Debian to move event plugins
    back onto the main library patch, which fixes problems with monitoring
    failing for snapshots and raid volumes. Closes: #807279, LP: #1556451,
    LP: #1561228.

 -- Steve Langasek <email address hidden> Sat, 16 Apr 2016 00:06:53 -0700