CentOS7 uses systemd for service management. When upgrading from CentOS6,
we need to be sure that certain services (systemd units) are enabled
or prepared for on-demand activation.

The LVM2 monitoring service inherits the old state, which means that if
the 'lvm2-monitor' service was enabled in CentOS6, it will also be enabled
in CentOS7 (lvm2-monitor.service) and vice versa.

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

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

The lvm2-lvmetad.socket is used for on-demand activation of lvm2-lvmetad.service
which starts the lvmetad - LVM metadata daemon that is used to cache LVM
metadata so LVM commands don't need to scan devices all the time and they
can reuse cached metadata. This functionality is used by default in CentOS7.