2. Synchronize time across the cluster

chrony is a flexible Network Time Protocol (NTP) implementation designed for synchronizing system clocks.

2.1. Chrony server installation

The master node will serve as NTP server for the rest of the cluster.

By default, chrony is installed in Almalinux but check if it is there
```
[root@master ~]# yum install chrony
```

Edit the /etc/chrony.conf file

By default, chrony does not allow other machines to synchronize their time with it. To make your master node serve as a time source for your cluster, you need to allow access from your cluster’s subnets:

# Allow NTP client access from local network.
allow 192.168.0.0/20
allow 192.168.16.0/20

The following lines ensure that the NTP server uses the Global NTP Pool Project servers as reference clocks.

# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

To serve time even when the master node is not synchronized with upstream NTP servers, uncomment the “local stratum” line
```
# Serve time even if not synchronized to a time source.
local stratum 10
```
This makes the server act as a stratum 8 time source when it can’t reach its upstream servers

Enable/start or restart the chronyd server. First check if it is arealdy running:

[root@master ~]# systemctl status chronyd
● chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2025-05-29 04:15:26 EDT; 3min 33s ago

If it is running, like in the previous example, restart it with the command:

[root@master ~]# systemctl restart chronyd

Otherwise, if it is not running, enable and start with service:

[root@master ~]# systemctl enable chronyd
[root@master ~]# systemctl start chronyd

Check if chrony is operating correctly and synchronized with its time sources

Use the chronyc tracking command to show detailed information about the synchronization status.:

[root@master ~]# chronyc tracking
Reference ID    : AC100101 (_gateway)
Stratum         : 4
Ref time (UTC)  : Wed Apr 09 03:23:29 2025
System time     : 0.000009234 seconds slow of NTP time
Last offset     : -0.000012674 seconds
RMS offset      : 0.000044795 seconds
Frequency       : 27.100 ppm slow

You can also check which time sources your server with the chronyc sources command:

2.2. Chrony Client Installation

Each compute node will act as a Chrony client. They run the same daemon as the server, however, only to synchronize their clocks with our own local Chrony server.

Warning

Before you can do any software installation, delete all Alma Linux repo files in /etc/yum.repos.d/.

[root@c01 ~]# rm /etc/yum.repos.d/Alma*

Check if the chrony daemon is installed

[root@c01 ~]# yum list installed | grep chrony
chrony.x86_64                         4.5-1.el8                        @anaconda

Edit the /etc/chrony.conf file

Add the following line to the Chrony configuration file
```
server ntp.hpc iburst
```
And remove or comment all other server or pool lines.

Warning

Verify you can resolve the name ntp.hpc i.e. host ntp.hpc. If the name is not resolvable you need to edit your DNS configuration to add it. Also remember, the host command is provided by bind-utils install it using yum. Alternatively, you can use the command ping.

Enable and start or restart the chronyd daemon

Verify your NTP client is synchronized with the reference clock from ntp.hpc

Use the chronyc sources command to query the current status of your system:

[root@c01 ~]# chronyc sources
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
^* master.hpc                    4   6   377     7   -141us[ -154us] +/-   13ms

From the master node, it is possible to check the list of NTP clients using the chronyc clients command:

[root@master ~]# chronyc clients
Hostname                      NTP   Drop Int IntL Last     Cmd   Drop Int  Last
===============================================================================
c01.hpc                         5      0   4   -    39       0      0   -     -
c02.hpc                         4      0   1   -    32       0      0   -     -