Use dnf with CentOS

date:2017-07-28 00:00
tags:centos, dnf, packaging

Blueprint: Use dnf with CentOS

CentOS 7 currently uses yum as its default package manager. However, Fedora has moved to dnf for several releases and it provides significant performance benefits. It can make the metadata cache, evaluate dependencies, and handle fastest mirror checks much more efficiently.

The dnf and yum package managers can co-exist together without causing conflicts. Several Fedora releases ran both of these simultaneously. The dnf packages are available in the EPEL repositories (which we currently enable). It uses all of the existing yum repositories and GPG keys as well.

Problem description

The CentOS gate jobs are notoriously slow and the integrated gate times out on tempest runs frequently. The longest running tasks in each role involve the installation of distro packages because these tasks use state: latest the yum tasks.

When Ansible sees state: latest, it goes through a fairly tedious process:

  • Run check-update, which checks the entire system for updates.
  • If some packages are returned (they need updates), Ansible searches the list to see if any packages from the yum task are in that list.
  • If some packages need updates, Ansible calls yum to install those packages.

This process can take 5-8 seconds even for one package. In comparison, dnf completes the task in 0.8-1.6 seconds. This should give us some wiggle room to get CI jobs completed sooner and convert more of the CentOS jobs from non-voting to voting.

Proposed change

On CentOS systems, we should install dnf and python-dnf (for Ansible compatibility). Ansible will prefer dnf over yum, so we would need to ensure that each role has support for dnf tasks. Since both package managers are interchangeable, this could be done by symlinking the *_install_dnf.yml task files to *_install_yum.yml and using the package module in those task files.

Alternatives

If dnf isn’t preferred, we could avoid using state: latest for CentOS installations. This would cause CentOS deployments to diverge from Ubuntu and OpenSUSE deployments and it would make bug triage more challenging.

Another option is to update the entire system when state: latest is provided but switch all of the package installation tasks to use state: present. This will save us a small amount of time since Ansible will skip the check-update step and go straight into updating all packages. This would be another diversion from the Ubuntu/OpenSUSE process, however.

Playbook/Role impact

Each role with a set of yum tasks would need to be converted to use package. A symlink would be needed so that CentOS systems with dnf installed would use the same tasks.

Upgrade impact

During the upgrade process, dnf would be installed on CentOS systems. Ansible would begin to use dnf, but the deployer could continue using yum for their own administration tasks if they prefer it.

Security impact

The dnf package manager supports the same configuration options as yum for checking GPG keys of packages and repositories.

Performance impact

The dnf package manager will provide better performance when managing packages, but the rest of the system will perform at the same levels.

End user impact

End users will not notice this change or gain any benefits from it.

Deployer impact

Deployers may notice that some roles use dnf while others use yum until all of the patches have merged. This won’t affect the running system, but it may make some playbooks faster than others.

Deployers would continue to deploy in the same ways that they currently do today.

Developer impact

Developers must be aware that dnf is present on CentOS systems and that Ansible will prefer it over yum. Any new roles/playbooks or updates to existing ones will need to include support for dnf via the dnf module or the package module (which selects dnf over yum already).

Dependencies

This spec is not dependent on any other spec or blueprint.

Implementation

Assignee(s)

Primary assignee:
Major Hayden (IRC: mhayden, Launchpad: rackerhacker)

Work items

  • Add dnf patches to the base roles first (openstack_hosts, lxc_hosts, etc)
  • Continue moving up the dependent roles until all roles include dnf-compatible tasks
  • Ensure that the integrated repository and openstack-ansible-tasks use dnf

Testing

The existing testing done in the OpenStack CI jobs will be sufficient for this work. If dnf is not installing packages properly or efficiently, we will see that reflected in the testing playbooks.

Documentation impact

This work will require some release notes to notify developers and deployers of the dnf change. However, there’s no need for extensive documentation since dnf supports the same configurations and arguments as yum.

References