Multiple CPU Architecture Support

date:2016-06-14 12:00
tags:openstack, ansible, power

The purpose of this spec is to enable OpenStack-Ansible to deploy OpenStack in clouds with multiple CPU architectures across the nodes to be deployed.

https://blueprints.launchpad.net/openstack-ansible/+spec/multi-arch-support

The purpose of this spec is to add support for multiple CPU architectures to OpenStack-Ansible. With the introduction of OpenStack services running on other architectures such as PPC64LE, OpenStack-Ansible needs to be extended to support environments where a mixed environment of CPU architectures exists.

Problem description

OpenStack-Ansible was initially built to support deployments to a single architecture, primarily x86. With the extension of OpenStack and OpenStack-Ansible support to other platforms such as POWER, support for deployments running a combination of different CPU architectures is needed.

For each deployment, OpenStack-Ansible creates and builds a ‘repo’ containing necessary artifacts for the OpenStack deployment. This repo holds components such as pip wheels, virtualenvs, and source trees for the different services to be deployed. For each deployment a single ‘master’ repo is designated where artifacts are built, then synchronized out to the rest of the slaves.

This creates problems in a multi-architecture deployment for nodes where the repo master’s CPU architecture is different than the architecture of other nodes. For example, deployment of a KVM on POWER compute node will fail when the artifacts were built on an x86 repo master used for the control plane.

Proposed change

Update OpenStack-Ansible and necessary roles to support building and deploying with multiple CPU architectures. This includes changes to:

  • Look at the Ansible facts for all hosts and determine the set of CPU architectures to build artifacts for.
  • Add support for assigning a build master for each CPU architecture.
  • Support building copies of CPU architecture specific artifacts on each build master, which will be synchronized to all slaves regardless of architecture.
  • Support tagging all architecture-specific build artifacts with the corresponding CPU architecture.
  • Ensure pre-built binaries used during installation (apt packages, etc) exist or are made available for the supported CPU architectures (x86, ppc64el) where possible, or documentation added to note limitations where not available.

Alternatives

  • Cross-compiling artifacts - This would remove the need for building, tagging and synchronizing between repos built on different architectures, but so far a reliable way to do this for wheels/venvs has not been found. This also introduces increased risk of architecture-specific cross-compiling issues as support for additional architectures is added.
  • Support only single-architecture OpenStack-Ansible deployments. This is harmful to deployers by limiting the potential integration of servers from other architectures into new and existing OpenStack-Ansible deployments

Playbook/Role impact

There will be impact to the repo playbooks to handle building and synchronizing across multiple architectures. There will also be minor impact to each role to support building and tagging artifacts with the required architecture information.

Upgrade impact

N/A

Security impact

N/A

Performance impact

Both repo build and synchronization operations will take longer to complete proportionate to the number of CPU architectures being deployed.

  • Repo builds will take longer as artifacts for each repo must be built serially to avoid collisions during synchronization of non-architecture specific packages.
  • Synchronizations will take longer and require more bandwidth to transfer when there are multiple architectures due to the increased number of artifacts being built.

End user impact

No end user impact is expected.

Deployer impact

Deployers will now be able to create deployments with servers of multiple CPU architectures included.

No other deployer impact is expected as the detection of multiple architectures, deploy of the required number of repo masters for each architecture, and build of artifacts for each architecture should happen dynamically.

Developer impact

Developer impact will be minor. Developers will now be able to develop for and test deployments across multiple CPU architectures. It also enables development of roles specific to other CPU architectures in the future, such as ARM.

Dependencies

N/A

Implementation

Assignee(s)

Primary assignee:
ashana@us.ibm.com/ashana
Other contributors:
adreznec@us.ibm.com/adreznec thorst@us.ibm.com/thorst

Work items

  • Add a repo discovery task to determine the CPU architectures used across all hosts, and assign and store facts about the master for each architecture.
  • Modify the repo-server playbook to support deploying a build master for each CPU architecture.
  • Modify the repo-build playbook to support building repos for each CPU architecture in serial.
  • Update the build tasks to support tagging each artifact with a corresponding CPU architecture.
  • Update the repo synchronization to support synchronizing artifacts from each build master out to all other slaves, regardless of CPU architecture.
  • Updates across tasks to use the new tagged artifact names.
  • Ensure binary packages are available on all package mirrors/locations for ppc64el in addition to i386/amd64, or that documentation is added to note where packages aren’t available.

Testing

A new test job will be added that deploys a two-node configuration, with one node belonging to the upstream/default architecture (x86) and the other of an additional CPU architecture to be supported (ppc64el, arm64, etc).

Documentation impact

Documentation covering how to configure multi-arch support will be added to the user guide.