Metadata Service Caching

The OpenStack Networking service proxies requests that VMs send to the Compute service to obtain their metadata. This functionality is provided by the neutron-metadata-agent or neutron-ovn-metadata-agent, depending on the ML2 backend used in the deployment. To obtain metadata from the Compute service, the instance ID needs to be sent to the nova-metadata-api. These two metadata agents provide the same functionality, but do it in slightly different ways, the difference being how the metadata agents find out the ID of the instance which is asking for metadata:

  • neutron-metadata-agent uses RPC to ask the neutron-server process for details about a port with a specific fixed IP address connected to the given network or router (proxy service is spawned for each Neutron router or Neutron network),

  • neutron-ovn-metadata-agent checks the instance ID in the port details of the OVN Southband DB.

For large scale deployments which are using the neutron-metadata-agent this may cause significant load on the RPC bus and neutron-server, since by default for each request to the metadata service (, the proxy will need to send an RPC query to retrieve the port details, and cloud-init is making many requests to this service during the VM boot process. To avoid this high load on the RPC bus, the neutron-metadata-agent allows using a caching mechanism for port details. Neutron uses oslo cache for this and it is configured through the following parameters in the cache section of the metadata_agent.ini file:

  • enabled: enables the caching mechanism.

  • backend: backend module to be used for caching.

  • expiration_time: TTL, in seconds, for cached items. In case of neutron-metadata-agent it is recommended to use some low value here, for example, 10 seconds. Usually cloud-init will make many requests to the metadata service in a short time during boot of a VM, so caching port details for just a few seconds should be enough to avoid many RPC requests. On the other hand, using too big a value may result in having cached details for a port which has already been deleted, as a fixed IP address can be quickly re-associated to a new port in Neutron.

The oslo.cache module provides many more configuration options which can be used to tune this caching mechanism. All of them are described in the oslo.cache documentation.