Time synchronization for ECS instances

Time synchronization in containers

No.

Do not run chronyd or ntpd inside a container. By default, containers share the host's system time and do not require separate time synchronization. Running a time synchronization service inside a container creates clock resource contention with the host and can severely impact application stability.

Manually synchronize time

Manually synchronizing time can cause a time jump and is not recommended. However, if the system time is off by a large amount (for example, several minutes or hours), you can perform a manual synchronization if your application can tolerate the sudden change.

Alibaba Cloud NTP server domains

The following table lists the Alibaba Cloud NTP server addresses for different networks. This service supports only IPv4.

• For ECS instances, use the VPC internal network domains for lower latency.

• For servers outside Alibaba Cloud, use the public network domains.

Service fee

Yes, the service is free of charge. Alibaba Cloud public images include a default time synchronization configuration, and ECS instances created from these images use either chrony or the ntpd service to synchronize time by default.

Upgrade from ntpd to chrony

Some older public images for ECS instances, such as CentOS 6, are configured to use the ntpd service. The ntpd project is no longer actively maintained. If your ECS instance uses the ntpd service and your applications do not depend on it, we recommend upgrading to chrony.

1. Uninstall the ntp service.

   Alibaba Cloud Linux/CentOS/RHEL

   sudo yum remove ntp

   Ubuntu/Debian

   sudo apt remove ntp

2. Install chrony.

   If the download fails, see How to resolve yum installation failures for chrony on CentOS 6 (EOL)?

   Alibaba Cloud Linux/CentOS/RHEL

   sudo yum install chrony

   Ubuntu/Debian

   sudo apt update
   
   sudo apt install chrony

3. Configure chrony.

   Edit the configuration file.

   Alibaba Cloud Linux/CentOS/RHEL

   sudo vim /etc/chrony.conf

   Ubuntu/Debian

   sudo vim /etc/chrony/chrony.conf

   In this file, you can add NTP servers and other settings. The following is a simple chrony.conf example:

   # NTP server settings
   server ntp.cloud.aliyuncs.com minpoll 4 maxpoll 10 iburst
   # Specify the location of the drift file.
   driftfile /var/lib/chrony/drift
   # Allow chronyd to be queried from localhost.
   allow 127.0.0.1

   The following table describes the parameters in the configuration file.

   Parameter	Description
   server ntp.cloud.aliyuncs.com	The domain name of the NTP server. chrony connects to this server to synchronize time.
   minpoll 4	Specifies the minimum polling interval in seconds, as a power of 2. For example, minpoll 4 sets the interval to 2⁴, or 16 seconds. This means chrony sends a synchronization request to the server at least every 16 seconds.
   maxpoll 10	Specifies the maximum polling interval in seconds, as a power of 2. For example, maxpoll 10 sets the interval to 2¹⁰, or 1024 seconds.
   iburst	This option speeds up the initial time synchronization. When chrony starts, it sends a burst of requests (typically eight) to quickly get an accurate time from the server.

4. Start the chrony service.

   1. Start the chrony service and enable it to start on boot.

      # Start the chrony service.
      sudo service chronyd start
      
      # Set chrony to start automatically when the system boots.
      sudo chkconfig chronyd on

   2. Check the chrony status. Use the chronyc tool to check the status and synchronization of chrony.

      # Check the service status.
      sudo service chronyd status

   3. Verify the time synchronization. Run the command to check the primary server status and the offset between the local clock and the time server.

      sudo chronyc tracking

Resolve yum installation failures for chrony on CentOS 6

1. Back up and remove the old repo configurations.

   sudo mkdir -p /etc/yum.repos.d/backup && sudo mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/backup/

2. Create a new repository file that points to the official CentOS Vault. Run the following command to create a new CentOS-Vault.repo file.

   sudo tee /etc/yum.repos.d/CentOS-Vault.repo <<'EOF'
   [C6.10-base]
   name=CentOS-6.10 - Base
   baseurl=http://vault.centos.org/6.10/os/$basearch/
   gpgcheck=1
   gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
   enabled=1
   [C6.10-updates]
   name=CentOS-6.10 - Updates
   baseurl=http://vault.centos.org/6.10/updates/$basearch/
   gpgcheck=1
   gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
   enabled=1
   [C6.10-extras]
   name=CentOS-6.10 - Extras
   baseurl=http://vault.centos.org/6.10/extras/$basearch/
   gpgcheck=1
   gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
   enabled=1
   EOF

3. Clean the yum cache and rebuild it.

   sudo yum clean all && sudo yum makecache

4. Install chrony.

   sudo yum install chrony