Cross-cell resize


This document describes how to configure nova for cross-cell resize. For information on same-cell resize, refer to Resize. For information on the cells v2 feature, refer to Cells (v2).

Historically resizing and cold migrating a server has been explicitly restricted to within the same cell in which the server already exists. The cross-cell resize feature allows configuring nova to allow resizing and cold migrating servers across cells.

The full design details are in the Ussuri spec and there is a video from a summit talk with a high-level overview.

Use case

There are many reasons to use multiple cells in a nova deployment beyond just scaling the database and message queue. Cells can also be used to shard a deployment by hardware generation and feature functionality. When sharding by hardware generation, it would be natural to setup a host aggregate for each cell and map flavors to the aggregate. Then when it comes time to decommission old hardware the deployer could provide new flavors and request that users resize to the new flavors, before some deadline, which under the covers will migrate their servers to the new cell with newer hardware. Administrators could also just cold migrate the servers during a maintenance window to the new cell.


To enable cross-cell resize functionality the following conditions must be met.

Minimum compute versions

All compute services must be upgraded to 21.0.0 (Ussuri) or later and not be pinned to older RPC API versions in upgrade_levels.compute.

Policy configuration

The policy rule compute:servers:resize:cross_cell controls who can perform a cross-cell resize or cold migrate operation. By default the policy disables the functionality for all users. A microversion is not required to opt into the behavior, just passing the policy check. As such, it is recommended to start by allowing only certain users to be able to perform a cross-cell resize or cold migration, for example by setting the rule to rule:admin_api or some other rule for test teams but not normal users until you are comfortable supporting the feature.

Compute driver

There are no special compute driver implementations required to support the feature, it is built on existing driver interfaces used during resize and shelve/unshelve. However, only the libvirt compute driver has integration testing in the nova-multi-cell CI job.


The networking API must expose the Port Bindings Extended API extension which was added in the 13.0.0 (Rocky) release for Neutron.


The types of events and their payloads remain unchanged. The major difference from same-cell resize is the publisher_id may be different in some cases since some events are sent from the conductor service rather than a compute service. For example, with same-cell resize the instance.resize_revert.start notification is sent from the source compute host in the finish_revert_resize method but with cross-cell resize that same notification is sent from the conductor service.

Obviously the actual message queue sending the notifications would be different for the source and target cells assuming they use separate transports.

Instance actions

The overall instance actions named resize, confirmResize and revertResize are the same as same-cell resize. However, the events which make up those actions will be different for cross-cell resize since the event names are generated based on the compute service methods involved in the operation and there are different methods involved in a cross-cell resize. This is important for triage when a cross-cell resize operation fails.


The CrossCellWeigher is enabled by default. When a scheduling request allows selecting compute nodes from another cell the weigher will by default prefer hosts within the source cell over hosts from another cell. However, this behavior is configurable using the filter_scheduler.cross_cell_move_weight_multiplier configuration option if, for example, you want to drain old cells when resizing or cold migrating.

Code flow

The end user experience is meant to not change, i.e. status transitions. A successfully cross-cell resized server will go to VERIFY_RESIZE status and from there the user can either confirm or revert the resized server using the normal confirmResize and revertResize server action APIs.

Under the covers there are some differences from a traditional same-cell resize:

  • There is no inter-compute interaction. Everything is synchronously orchestrated from the (super)conductor service. This uses the long_rpc_timeout configuration option.

  • The orchestration tasks in the (super)conductor service are in charge of creating a copy of the instance and its related records in the target cell database at the beginning of the operation, deleting them in case of rollback or when the resize is confirmed/reverted, and updating the instance_mappings table record in the API database.

  • Non-volume-backed servers will have their root disk uploaded to the image service as a temporary snapshot image just like during the shelveOffload operation. When finishing the resize on the destination host in the target cell that snapshot image will be used to spawn the guest and then the snapshot image will be deleted.

Sequence diagram

The following diagrams are current as of the 21.0.0 (Ussuri) release.


This is the sequence of calls to get the server to VERIFY_RESIZE status.

blockdiag API Conductor Scheduler Source Destination resize_instance/migrate_server select_destinations prep_snapshot_based_resize_at_dest prep_snapshot_based_resize_at_source finish_snapshot_based_resize_at_dest update instance mapping cast MigrationTask TargetDBSetupTask PrepResizeAtDestTask PrepResizeAtSourceTask FinishResizeAtDestTask FinishResizeAtDestTask

Confirm resize

This is the sequence of calls when confirming or deleting a server in VERIFY_RESIZE status.

blockdiag API Conductor Source confirm_snapshot_based_resize confirm_snapshot_based_resize_at_source hard delete source cell instance update target cell instance status cast (or call if deleting) call ConfirmResizeTask

Revert resize

This is the sequence of calls when reverting a server in VERIFY_RESIZE status.

blockdiag API Conductor Source Destination revert_snapshot_based_resize update records from target to source cell update instance mapping revert_snapshot_based_resize_at_dest hard delete target cell instance finish_revert_snapshot_based_resize_at_source cast call call RevertResizeTask


These are known to not yet be supported in the code:

  • Instances with ports attached that have bandwidth-aware resource provider allocations. Nova falls back to same-cell resize if the server has such ports.

  • Rescheduling to alternative hosts within the same target cell in case the primary selected host fails the prep_snapshot_based_resize_at_dest call.

These may not work since they have not been validated by integration testing:

  • Instances with PCI devices attached.

  • Instances with a NUMA topology.

Other limitations:

  • The config drive associated with the server, if there is one, will be re-generated on the destination host in the target cell. Therefore if the server was created with personality files they will be lost. However, this is no worse than evacuating a server that had a config drive when the source and destination compute host are not on shared storage or when shelve offloading and unshelving a server with a config drive. If necessary, the resized server can be rebuilt to regain the personality files.

  • The _poll_unconfirmed_resizes periodic task, which can be configured to automatically confirm pending resizes on the target host, might not support cross-cell resizes because doing so would require an up-call to the API to confirm the resize and cleanup the source cell database.



Configure a service user in case the user token times out, e.g. during the snapshot and download of a large server image.

If RPC calls are timing out with a MessagingTimeout error in the logs, check the long_rpc_timeout option to see if it is high enough though the default value (30 minutes) should be sufficient.

Recovering from failure

The orchestration tasks in conductor that drive the operation are built with rollbacks so each part of the operation can be rolled back in order if a subsequent task fails.

The thing to keep in mind is the instance_mappings record in the API DB is the authority on where the instance “lives” and that is where the API will go to show the instance in a GET /servers/{server_id} call or any action performed on the server, including deleting it.

So if the resize fails and there is a copy of the instance and its related records in the target cell, the tasks should automatically delete them but if not you can hard-delete the records from whichever cell is not the one in the instance_mappings table.

If the instance is in ERROR status, check the logs in both the source and destination compute service to see if there is anything that needs to be manually recovered, for example volume attachments or port bindings, and also check the (super)conductor service logs. Assuming volume attachments and port bindings are OK (current and pointing at the correct host), then try hard rebooting the server to get it back to ACTIVE status. If that fails, you may need to rebuild the server on the source host. Note that the guest’s disks on the source host are not deleted until the resize is confirmed so if there is an issue prior to confirm or confirm itself fails, the guest disks should still be available for rebuilding the instance if necessary.