Red Hat Enterprise Linux 7 uses systemd for service management. When upgrading from Red Hat Enterprise Linux 6,
we need to be sure that certain services (systemd units) are enabled
or prepared for the on-demand activation.

The LVM2 monitoring service inherits the old state, which means that if
the 'lvm2-monitor' service was enabled in Red Hat Enterprise Linux 6, it will also be enabled
in Red Hat Enterprise Linux 7 (lvm2-monitor.service) and vice versa.

In addition, these systemd units are enabled to allow for the on-demand
service activation:
  dm-event.socket
  lvm2-lvmetad.socket

The dm-event.socket is used for the on-demand activation of the dm-event.service.
This is an essential part of the device-mapper monitoring feature (which also
covers monitoring of the LVM devices).

The lvm2-lvmetad.socket is used for the on-demand activation of the lvm2-lvmetad.service
which starts lvmetad - the LVM metadata daemon that is used to cache LVM
metadata so that the LVM commands would not have to scan devices all the time and they
could reuse the cached metadata. This functionality is used by default in Red Hat Enterprise Linux 7.
