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

The LVM2 monitoring service inherits the original 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 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), which is used to cache LVM
metadata so that the LVM commands do not have to scan devices all the time, and so that they
can reuse the cached metadata. This functionality is used by default in Red Hat Enterprise Linux 7.
