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High Availability and Scalability

HA and Scalability #

ironic-api #

The Bare Metal API service is stateless, and thus can be easily scaled horizontally. It is recommended to deploy it as a WSGI application behind e.g. Apache or another WSGI container.

Note

This service accesses the ironic database for reading entities (e.g. in response to GET /v1/nodes request) and in rare cases for writing.

ironic-conductor #

High availability #

The Bare Metal conductor service utilizes the active/active HA model. Every conductor manages a certain subset of nodes. The nodes are organized in a hash ring that tries to keep the load spread more or less uniformly across the conductors. When a conductor is considered offline, its nodes are taken over by other conductors. As a result of this, you need at least 2 conductor hosts for an HA deployment.

Performance #

Conductors can be resource intensive, so it is recommended (but not required) to keep all conductors separate from other services in the cloud. The minimum required number of conductors in a deployment depends on several factors:

  • the performance of the hardware where the conductors will be running,
  • the speed and reliability of the management controller of the bare metal nodes (for example, handling slower controllers may require having less nodes per conductor),
  • the frequency, at which the management controllers are polled by the Bare Metal service (see the sync_power_state_interval option),
  • the bare metal driver used for nodes (see Hardware and drivers above),
  • the network performance,
  • the maximum number of bare metal nodes that are provisioned simultaneously (see the max_concurrent_builds option for the Compute service).

We recommend a target of 100 bare metal nodes per conductor for maximum reliability and performance. There is some tolerance for a larger number per conductor. However, it was reported [1] [2] that reliability degrades when handling approximately 300 bare metal nodes per conductor.

Disk space #

Each conductor needs enough free disk space to cache images it uses. Depending on the combination of the deploy interface and the boot option, the space requirements are different:

  • The deployment kernel and ramdisk are always cached during the deployment.
  • When [agent]image_download_source is set to http and Glance is used, the conductor will download instances images locally to serve them from its HTTP server. Use swift to publish images using temporary URLs and convert them on the node’s side.When [agent]image_download_source is set to local, it will happen even for HTTP(s) URLs. For standalone case use http to avoid unnecessary caching of images.In both cases a cached image is converted to raw if force_raw_images is True (the default).See Deploy with custom HTTP servers and Streaming raw images for more details.
  • When network boot is used, the instance image kernel and ramdisk are cached locally while the instance is active.
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