15.4. Considerations for multi-cell deployments

Maximum number of Compute nodes in a multi-cell deployment
The maximum number of Compute nodes is 500 across all cells.
Cross-cell instance migrations

Migrating an instance from a host in one cell to a host in another cell is not supported. This limitation affects the following operations:

  • cold migration
  • live migration
  • unshelve
  • resize
  • evacuation
Service quotas

Compute service quotas are calculated dynamically at each resource consumption point, instead of statically in the database. In multi-cell deployments, unreachable cells cannot provide usage information in real-time, which might cause the quotas to be exceeded when the cell is reachable again.

You can use the Placement service and API database to configure the quota calculation to withstand failed or unreachable cells.

API database
The Compute API database is always global for all cells and cannot be duplicated for each cell.
Console proxies
You must configure console proxies for each cell, because console token authorizations are stored in cell databases. Each console proxy server needs to access the database.connection information of the corresponding cell database.
Compute metadata API

You can run the Compute metadata API globally or in each cell. Choose one of the following:

  • If you have networks that cover multiple cells, you need to run the metadata API globally so that it can bridge between the cells. In this case, the metadata API needs to access the api_database.connection information.
  • If you have networks in separate segments for each cell, you can run the metadata API separately in each cell. This configuration can improve performance and data isolation. In this case, neutron-metadata-agent service point to the corresponding nova-api-metadata service.

You use the api.local_metadata_per_cell configuration option to set which method to implement. For details on configuring this option, see the Create environment files with cell parameters section in Deploying a multi-cell overcloud.