This section describes the various recommended practices/tips for runnning and maintaining CellsV2 for admins and operators. For more details regarding the basic concept of CellsV2 and its layout please see the main Cells Layout (v2) page.
Handling cell failures¶
For an explanation on how
nova-api handles cell failures please see the
Handling Down Cells
section of the Compute API guide. Below, you can find some recommended practices and
considerations for effectively tolerating cell failure situations.
Since a cell being reachable or not is determined through timeouts, it is suggested to provide suitable values for the following settings based on your requirements.
database.max_retriesis 10 by default meaning every time a cell becomes unreachable, it would retry 10 times before nova can declare the cell as a “down” cell.
database.retry_intervalis 10 seconds and
oslo_messaging_rabbit.rabbit_retry_intervalis 1 second by default meaning every time a cell becomes unreachable it would retry every 10 seconds or 1 second depending on if it’s a database or a message queue problem.
Nova also has a timeout value called
CELL_TIMEOUTwhich is hardcoded to 60 seconds and that is the total time the nova-api would wait before returning partial results for the “down” cells.
The values of the above settings will affect the time required for nova to decide if a cell is unreachable and then take the necessary actions like returning partial results.
The operator can also control the results of certain actions like listing
servers and services depending on the value of the
api.list_records_by_skipping_down_cells config option.
If this is true, the results from the unreachable cells will be skipped
and if it is false, the request will just fail with an API error in situations where
partial constructs cannot be computed.
Disabling down cells¶
While the temporary outage in the infrastructure is being fixed, the affected cells can be disabled so that they are removed from being scheduling candidates. To enable or disable a cell, use nova-manage cell_v2 update_cell --cell_uuid <cell_uuid> --disable. See the Cells v2 Commands man page for details on command usage.
Services and Performance: In case a cell is down during the startup of nova services, there is the chance that the services hang because of not being able to connect to all the cell databases that might be required for certain calculations and initializations. An example scenario of this situation is if
upgrade_levels.computeis set to
nova-apiservice hangs on startup if there is at least one unreachable cell. This is because it needs to connect to all the cells to gather information on each of the compute service’s version to determine the compute version cap to use. The current workaround is to pin the
upgrade_levels.computeto a particular version like “rocky” and get the service up under such situations. See bug 1815697 for more details. Also note that in general during situations where cells are not reachable certain “slowness” may be experienced in operations requiring hitting all the cells because of the aforementioned configurable timeout/retry values.
Counting Quotas: Another known issue is in the current approach of counting quotas where we query each cell database to get the used resources and aggregate them which makes it sensitive to temporary cell outages. While the cell is unavailable, we cannot count resource usage residing in that cell database and things would behave as though more quota is available than should be. That is, if a tenant has used all of their quota and part of it is in cell A and cell A goes offline temporarily, that tenant will suddenly be able to allocate more resources than their limit (assuming cell A returns, the tenant will have more resources allocated than their allowed quota).
Starting in the Train (20.0.0) release, it is possible to configure counting of quota usage from the placement service and API database to make quota usage calculations resilient to down or poor-performing cells in a multi-cell environment. See the quotas documentation for more details.