Time Synchronization
--------------------
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The {pve} cluster stack itself relies heavily on the fact that all
the nodes have precisely synchronized time. Some other components,
like Ceph, also won't work properly if the local time on all nodes is
not in sync.

Time synchronization between nodes can be achieved using the ``Network
Time Protocol'' (`NTP`). As of {pve} 7, `chrony` is used as the default
NTP daemon, while {pve} 6 uses `systemd-timesyncd`. Both come preconfigured to
use a set of public servers.

IMPORTANT: If you upgrade your system to {pve} 7, it is recommended that you
manually install either `chrony`, `ntp` or `openntpd`.

Using Custom NTP Servers
~~~~~~~~~~~~~~~~~~~~~~~~

In some cases, it might be desired to use non-default NTP
servers. For example, if your {pve} nodes do not have access to the
public internet due to restrictive firewall rules, you
need to set up local NTP servers and tell the NTP daemon to use
them.

For systems using chrony:
^^^^^^^^^^^^^^^^^^^^^^^^^^

Specify which servers `chrony` should use in `/etc/chrony/chrony.conf`:

----
server ntp1.example.com iburst
server ntp2.example.com iburst
server ntp3.example.com iburst
----

Restart `chrony`:

 # systemctl restart chronyd

Check the journal to confirm that the newly configured NTP servers are being
used:

 # journalctl --since -1h -u chrony

----
...
Aug 26 13:00:09 node1 systemd[1]: Started chrony, an NTP client/server.
Aug 26 13:00:15 node1 chronyd[4873]: Selected source 10.0.0.1 (ntp1.example.com)
Aug 26 13:00:15 node1 chronyd[4873]: System clock TAI offset set to 37 seconds
...
----

For systems using systemd-timesyncd:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Specify which servers `systemd-timesyncd` should use in
`/etc/systemd/timesyncd.conf`:

----
[Time]
NTP=ntp1.example.com ntp2.example.com ntp3.example.com ntp4.example.com
----

Then, restart the synchronization service (`systemctl restart
systemd-timesyncd`), and verify that your newly configured NTP servers are in
use by checking the journal (`journalctl --since -1h -u systemd-timesyncd`):

----
...
Oct 07 14:58:36 node1 systemd[1]: Stopping Network Time Synchronization...
Oct 07 14:58:36 node1 systemd[1]: Starting Network Time Synchronization...
Oct 07 14:58:36 node1 systemd[1]: Started Network Time Synchronization.
Oct 07 14:58:36 node1 systemd-timesyncd[13514]: Using NTP server 10.0.0.1:123 (ntp1.example.com).
Oct 07 14:58:36 node1 systemd-timesyncd[13514]: interval/delta/delay/jitter/drift 64s/-0.002s/0.020s/0.000s/-31ppm
...
----