CentOS In-Place Upgrade

CentOS 8.0 was released on September 24th, 2019 and 8.1 on January 15, 2020. This article describes how a CentOS 7 may be upgraded in place. The steps are captured in an Ansible role published on GitHub.

Theory of Operation

The steps to migrate a CentOS 7 instance to CentOS 8.1 are:

1. Replace yum with dnf

   a. Prepare yum installation

   b. Install dnf

   c. Remove yum

2. Use dnf to upgrade

   a. Configure CentOS 8.1 packages

   b. Install CentOS 8.1 (userland)

   c. Install CentOS 8.1 kernel

Step #1 is necessitated because dnf has replaced yum in CentOS/RHEL 8 systems. The implementation is discussed in the next session.

Implementation

The following subsection describe the major steps of the upgrade.

Replace yum with dnf

The first step is to replace yum with dnf. The implementation uses the fact that /usr/bin/yum is a regular file when yum is installed and a symbolic link to dnf-3 when dnf is installed.

- name: /usr/bin/yum
  stat: path=/usr/bin/yum
  register: yum

The condition yum.stat.isreg is defined and yum.stat.isreg, if true, indicates yum was the package manager when Anisble was invoked. The Ansible script takes advantage of this observation to provide idempotent operation. package-cleanup(1) is installed from epel-release and used to remove locally installed RPMs.

- name: epel-release
  package:
    name: epel-release
    state: latest
  when:
    - yum.stat.isreg is defined and yum.stat.isreg

- name: yum-utils
  package:
    name: yum-utils
    state: latest
  when:
    - yum.stat.isreg is defined and yum.stat.isreg

- name: package-cleanup
  command:
    cmd: "{{ item }}"
  loop:
    - package-cleanup --leaves
    - package-cleanup --orphans
  when:
    - yum.stat.isreg is defined and yum.stat.isreg

The author's use case is to upgrade a fresh install of CentOS 7. However, if the upgrade is to be performed on a configured system, then rpmconf should be invoked to determine if any configuration files need to be preserved and/or migrated:

# yum -y install rpmconf
# rpmconf -a

dnf is installed with yum and then yum is removed with the corresponding dnf request, /etc/yum is removed, and dnf is updated. It is critical that these steps are completed so the system is not left in an inconsistent state without a functioning yum or dnf.

- name: dnf
  package:
    name: dnf
    state: latest
  when:
    - yum.stat.isreg is defined and yum.stat.isreg

- name: yum -> dnf
  shell: |-
    dnf -y remove yum yum-metadata-parser
    rm -rf /etc/yum
    dnf -y upgrade
  args:
    warn: false
  when:
    - yum.stat.isreg is defined and yum.stat.isreg

dnf is now installed and available to use for an in-place upgrade.

Upgrade CentOS

CentOS 8 requires 3 CentOS RPMs plus the latest epel-release (obtained via RPM) which are installed explicitly with dnf. The conditional ansible_distribution_major_version is version(releasever, "lt") is leveraged to provide idempotent operation and avoid re-running once CentOS 8 is installed.

- name: centos_packages
  vars:
    target: 8.1-1.1911.0.8.el8
    arch: "{{ ansible_architecture }}"
    releasever: "{{ target | regex_replace('^([0-9]+)[.].*$', '\\1') }}"
    BaseOS: "http://mirror.centos.org/centos/{{ releasever }}/BaseOS"
    Packages: "{{ BaseOS }}/{{ arch }}/os/Packages"
  set_fact:
    releasever: "{{ releasever }}"
    centos_packages:
      - "{{ Packages }}/centos-gpg-keys-{{ target }}.noarch.rpm"
      - "{{ Packages }}/centos-release-{{ target }}.{{ arch }}.rpm"
      - "{{ Packages }}/centos-repos-{{ target }}.{{ arch }}.rpm"
      - "https://dl.fedoraproject.org/pub/epel/epel-release-latest-{{ releasever }}.noarch.rpm"

The system is now ready for the actual upgrade. The CentOS Upgrade script has to run until reboot or the system will be left in an inconsistent state. Unfortunately python is replaced (and moved) so the Anisble client loses communication during the process eliminating the possibility of using the Anisble reboot module. Instead, the administrator should reinvoke the Ansible play once the update and reboot are complete.

- name: warn
  debug:
    msg: >-
      Warning: CentOS Upgrade will install kernel and initiate reboot
  when:
    - ansible_distribution_major_version is version(releasever, "lt")

- name: CentOS Upgrade
  shell: |-
    dnf -y install {{ centos_packages | join(" ") }}
    dnf clean all
    rpm -e $(rpm -q kernel)
    rpm -e --nodeps sysvinit-tools
    dnf -y --releasever={{ releasever }} --allowerasing --setopt=deltarpm=false distro-sync
    dnf -y install kernel-core
    dnf -y groupupdate "Core" "Minimal Install"
    shutdown -r now
  args:
    warn: false
  when:
    - ansible_distribution_major_version is version(releasever, "lt")

Post Upgrade Steps

The script allows for enabling the CentOS-Plus repository and updating installed packages after the reboot.

- name: Enable CentOS-Plus repository
  ini_file:
    dest: /etc/yum.repos.d/CentOS-centosplus.repo
    create: no
    section: centosplus
    option: enabled
    value: "1"

- name: package update
  package:
    name: "*"
    state: latest

Summary

CentOS 7 installation may be upgraded to CentOS 8 in-place once yum is replaced by dnf.