52 lines
2.6 KiB
ReStructuredText
52 lines
2.6 KiB
ReStructuredText
.. _config-metadata-caching:
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========================
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Metadata Service Caching
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========================
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The OpenStack Networking service proxies requests that VMs send to the
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Compute service to obtain their metadata. This functionality is provided by the
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``neutron-metadata-agent`` or ``neutron-ovn-metadata-agent``, depending on the
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ML2 backend used in the deployment.
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To obtain metadata from the Compute service, the instance ID needs to be sent
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to the ``nova-metadata-api``.
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These two metadata agents provide the same functionality, but do it
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in slightly different ways, the difference being how the metadata agents find
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out the ID of the instance which is asking for metadata:
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* ``neutron-metadata-agent`` uses RPC to ask the neutron-server process for
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details about a port with a specific fixed IP address connected to the given
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network or router (proxy service is spawned for each Neutron router or
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Neutron network),
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* ``neutron-ovn-metadata-agent`` checks the instance ID in the port details of
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the OVN Southband DB.
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For large scale deployments which are using the ``neutron-metadata-agent`` this
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may cause significant load on the RPC bus and neutron-server, since by default
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for each request to the metadata service (``169.254.169.254``), the proxy will
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need to send an RPC query to retrieve the port details, and `cloud-init
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<https://cloudinit.readthedocs.io/>`_ is making many requests to this service
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during the VM boot process.
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To avoid this high load on the RPC bus, the ``neutron-metadata-agent`` allows
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using a caching mechanism for port details.
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Neutron uses `oslo cache
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<https://docs.openstack.org/oslo.cache/latest/index.html>`_ for
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this and it is configured through the following parameters in the ``cache``
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section of the ``metadata_agent.ini`` file:
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* ``enabled``: enables the caching mechanism.
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* ``backend``: backend module to be used for caching.
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* ``expiration_time``: TTL, in seconds, for cached items. In case of
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``neutron-metadata-agent`` it is recommended to use some low value here, for
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example, 10 seconds. Usually cloud-init will make many requests to the
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metadata service in a short time during boot of a VM, so caching port details
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for just a few seconds should be enough to avoid many RPC requests. On the
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other hand, using too big a value may result in having cached details for a
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port which has already been deleted, as a fixed IP address can be quickly
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re-associated to a new port in Neutron.
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The oslo.cache module provides many more configuration options which can be
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used to tune this caching mechanism. All of them are described in the
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oslo.cache `documentation
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<https://docs.openstack.org/oslo.cache/latest/configuration/index.html>`_.
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