# Cells (v2)

Added in version 16.0.0: (Pike)

This document describes the layout of a deployment with cells v2, including deployment considerations for security and scale and recommended practices and tips for running and maintaining cells v2 for admins and operators. It is focused on code present in Pike and later, and while it is geared towards people who want to have multiple cells for whatever reason, the nature of the cells v2 support in Nova means that it applies in some way to all deployments.

Before reading any further, there is a nice overview presentation that Andrew Laski gave at the Austin (Newton) summit which may be worth watching.

Note

Cells v2 is different to the cells feature found in earlier versions of nova, also known as Cells v1. Cells v1 was deprecated in 16.0.0 (Pike) and removed entirely in Train (20.0.0).

## Overview

The purpose of the cells functionality in nova is to allow larger deployments to shard their many compute nodes into cells. All nova deployments are by definition cells deployments, even if most will only ever have a single cell. This means a multi-cell deployment will not be radically different from a “standard” nova deployment.

Consider such a deployment. It will consists of the following components:

- The Compute API which provides the external REST API to users.

- The nova-scheduler and `placement` services which are responsible for tracking resources and deciding which compute node instances should be on.

- An “API database” that is used primarily by the Compute API and nova-scheduler (called API-level services below) to track location information about instances, as well as a temporary location for instances being built but not yet scheduled.

- The nova-conductor service which offloads long-running tasks for the API-level services and insulates compute nodes from direct database access

- The nova-compute service which manages the virt driver and hypervisor host.

- A “cell database” which is used by API, conductor and compute services, and which houses the majority of the information about instances.

- A “cell0 database” which is just like the cell database, but contains only instances that failed to be scheduled. This database mimics a regular cell, but has no compute nodes and is used only as a place to put instances that fail to land on a real compute node (and thus a real cell).

- A message queue which allows the services to communicate with each other via RPC.

In smaller deployments, there will typically be a single message queue that all services share and a single database server which hosts the API database, a single cell database, as well as the required cell0 database. Because we only have one “real” cell, we consider this a “single-cell deployment”.

In larger deployments, we can opt to shard the deployment using multiple cells. In this configuration there will still only be one global API database but there will be a cell database (where the bulk of the instance information lives) for each cell, each containing a portion of the instances for the entire deployment within, as well as per-cell message queues and per-cell nova-conductor instances. There will also be an additional nova-conductor instance, known as a super conductor, to handle API-level operations.

In these larger deployments, each of the nova services will use a cell-specific configuration file, all of which will at a minimum specify a message queue endpoint (i.e. `transport_url`). Most of the services will also contain database connection configuration information (i.e. `database.connection`), while API-level services that need access to the global routing and placement information will also be configured to reach the API database (i.e. `api_database.connection`).

Note

The pair of `transport_url` and `database.connection` configured for a service defines what cell a service lives in.

API-level services need to be able to contact other services in all of the cells. Since they only have one configured `transport_url` and `database.connection`, they look up the information for the other cells in the API database, with records called cell mappings.

Note

The API database must have cell mapping records that match the `transport_url` and `database.connection` configuration options of the lower-level services. See the `nova-manage` Cells v2 Commands commands for more information about how to create and examine these records.

The following section goes into more detail about the difference between single-cell and multi-cell deployments.

## Service layout

The services generally have a well-defined communication pattern that dictates their layout in a deployment. In a small/simple scenario, the rules do not have much of an impact as all the services can communicate with each other on a single message bus and in a single cell database. However, as the deployment grows, scaling and security concerns may drive separation and isolation of the services.

### Single cell

This is a diagram of the basic services that a simple (single-cell) deployment would have, as well as the relationships (i.e. communication paths) between them:

digraph services { graph [pad="0.35", ranksep="0.65", nodesep="0.55", concentrate=true]; node [fontsize=10 fontname="Monospace"]; edge [arrowhead="normal", arrowsize="0.8"]; labelloc=bottom; labeljust=left; { rank=same api [label="nova-api"] apidb [label="API Database" shape="box"] scheduler [label="nova-scheduler"] } { rank=same mq [label="MQ" shape="diamond"] conductor [label="nova-conductor"] } { rank=same cell0db [label="Cell0 Database" shape="box"] celldb [label="Cell Database" shape="box"] compute [label="nova-compute"] } api -> mq -> compute conductor -> mq -> scheduler api -> apidb api -> cell0db api -> celldb conductor -> apidb conductor -> cell0db conductor -> celldb }

All of the services are configured to talk to each other over the same message bus, and there is only one cell database where live instance data resides. The cell0 database is present (and required) but as no compute nodes are connected to it, this is still a “single cell” deployment.

### Multiple cells

In order to shard the services into multiple cells, a number of things must happen. First, the message bus must be split into pieces along the same lines as the cell database. Second, a dedicated conductor must be run for the API-level services, with access to the API database and a dedicated message queue. We call this super conductor to distinguish its place and purpose from the per-cell conductor nodes.

digraph services2 { graph [pad="0.35", ranksep="0.65", nodesep="0.55", concentrate=true]; node [fontsize=10 fontname="Monospace"]; edge [arrowhead="normal", arrowsize="0.8"]; labelloc=bottom; labeljust=left; subgraph api { api [label="nova-api"] scheduler [label="nova-scheduler"] conductor [label="super conductor"] { rank=same apimq [label="API MQ" shape="diamond"] apidb [label="API Database" shape="box"] } api -> apimq -> conductor api -> apidb conductor -> apimq -> scheduler conductor -> apidb } subgraph clustercell0 { label="Cell 0" color=green cell0db [label="Cell Database" shape="box"] } subgraph clustercell1 { label="Cell 1" color=blue mq1 [label="Cell MQ" shape="diamond"] cell1db [label="Cell Database" shape="box"] conductor1 [label="nova-conductor"] compute1 [label="nova-compute"] conductor1 -> mq1 -> compute1 conductor1 -> cell1db } subgraph clustercell2 { label="Cell 2" color=red mq2 [label="Cell MQ" shape="diamond"] cell2db [label="Cell Database" shape="box"] conductor2 [label="nova-conductor"] compute2 [label="nova-compute"] conductor2 -> mq2 -> compute2 conductor2 -> cell2db } api -> mq1 -> conductor1 api -> mq2 -> conductor2 api -> cell0db api -> cell1db api -> cell2db conductor -> cell0db conductor -> cell1db conductor -> mq1 conductor -> cell2db conductor -> mq2 }

It is important to note that services in the lower cell boxes only have the ability to call back to the placement API but cannot access any other API-layer services via RPC, nor do they have access to the API database for global visibility of resources across the cloud. This is intentional and provides security and failure domain isolation benefits, but also has impacts on some things that would otherwise require this any-to-any communication style. Check Operations requiring upcalls below for the most up-to-date information about any caveats that may be present due to this limitation.

## Database layout

As mentioned previously, there is a split between global data and data that is local to a cell. These databases schema are referred to as the API and main database schemas, respectively.

### API database

The API database is the database used for API-level services, such as the Compute API and, in a multi-cell deployment, the superconductor. The models and migrations related to this database can be found in `nova.db.api`, and the database can be managed using the nova-manage api_db commands.

### Main (cell-level) database

The main database is the database used for cell-level nova-conductor instances. The models and migrations related to this database can be found in `nova.db.main`, and the database can be managed using the nova-manage db commands.

## Usage

As noted previously, all deployments are in effect now cells v2 deployments. As a result, setup of any nova deployment - even those that intend to only have one cell - will involve some level of cells configuration. These changes are configuration-related, both in the main nova configuration file as well as some extra records in the databases.

All nova deployments must now have the following databases available and configured:

1. The “API” database

2. One special “cell” database called “cell0”

3. One (or eventually more) “cell” databases

Thus, a small nova deployment will have an API database, a cell0, and what we will call here a “cell1” database. High-level tracking information is kept in the API database. Instances that are never scheduled are relegated to the cell0 database, which is effectively a graveyard of instances that failed to start. All successful/running instances are stored in “cell1”.

Note

Since Nova services make use of both configuration file and some databases records, starting or restarting those services with an incomplete configuration could lead to an incorrect deployment. Only restart the services once you are done with the described steps below.

Note

The following examples show the full expanded command line usage of the setup commands. This is to make it easier to visualize which of the various URLs are used by each of the commands. However, you should be able to put all of that in the config file and nova-manage will use those values. If need be, you can create separate config files and pass them as `nova-manage --config-file foo.conf` to control the behavior without specifying things on the command lines.

### Configuring a new deployment

If you are installing Nova for the first time and have no compute hosts in the database yet then it will be necessary to configure cell0 and at least one additional “real” cell. To begin, ensure your API database schema has been populated using the nova-manage api_db sync command. Ensure the connection information for this database is stored in the `nova.conf` file using the `api_database.connection` config option:

    [api_database]
    connection = mysql+pymysql://root:secretmysql@dbserver/nova_api?charset=utf8

Since there may be multiple “cell” databases (and in fact everyone will have cell0 and cell1 at a minimum), connection info for these is stored in the API database. Thus, the API database must exist and must provide information on how to connect to it before continuing to the steps below, so that nova-manage can find your other databases.

Next, we will create the necessary records for the cell0 database. To do that we will first use nova-manage cell_v2 map_cell0 to create and map cell0. For example:

    $ nova-manage cell_v2 map_cell0 \
        --database_connection mysql+pymysql://root:secretmysql@dbserver/nova_cell0?charset=utf8

Note

If you don’t specify `--database_connection` then the commands will use the `database.connection` value from your config file and mangle the database name to have a `_cell0` suffix

Warning

If your databases are on separate hosts then you should specify `--database_connection` or make certain that the `nova.conf` being used has the `database.connection` value pointing to the same user/password/host that will work for the cell0 database. If the cell0 mapping was created incorrectly, it can be deleted using the nova-manage cell_v2 delete_cell command before running nova-manage cell_v2 map_cell0 again with the proper database connection value.

We will then use nova-manage db sync to apply the database schema to this new database. For example:

    $ nova-manage db sync \
        --database_connection mysql+pymysql://root:secretmysql@dbserver/nova_cell0?charset=utf8

Since no hosts are ever in cell0, nothing further is required for its setup. Note that all deployments only ever have one cell0, as it is special, so once you have done this step you never need to do it again, even if you add more regular cells.

Now, we must create another cell which will be our first “regular” cell, which has actual compute hosts in it, and to which instances can actually be scheduled. First, we create the cell record using nova-manage cell_v2 create_cell. For example:

    $ nova-manage cell_v2 create_cell \
        --name cell1 \
        --database_connection  mysql+pymysql://root:secretmysql@127.0.0.1/nova?charset=utf8 \
        --transport-url rabbit://stackrabbit:secretrabbit@mqserver:5672/

Note

If you don’t specify the database and transport urls then nova-manage will use the `transport_url` and `database.connection` values from the config file.

Note

It is a good idea to specify a name for the new cell you create so you can easily look up cell UUIDs with the nova-manage cell_v2 list_cells command later if needed.

Note

The nova-manage cell_v2 create_cell command will print the UUID of the newly-created cell if `--verbose` is passed, which is useful if you need to run commands like nova-manage cell_v2 discover_hosts targeted at a specific cell.

At this point, the API database can now find the cell database, and further commands will attempt to look inside. If this is a completely fresh database (such as if you’re adding a cell, or if this is a new deployment), then you will need to run nova-manage db sync on it to initialize the schema.

Now we have a cell, but no hosts are in it which means the scheduler will never actually place instances there. The next step is to scan the database for compute node records and add them into the cell we just created. For this step, you must have had a compute node started such that it registers itself as a running service. You can identify this using the openstack compute service list command:

    $ openstack compute service list --service nova-compute

Once that has happened, you can scan and add it to the cell using the nova-manage cell_v2 discover_hosts command:

    $ nova-manage cell_v2 discover_hosts

This command will connect to any databases for which you have created cells (as above), look for hosts that have registered themselves there, and map those hosts in the API database so that they are visible to the scheduler as available targets for instances. Any time you add more compute hosts to a cell, you need to re-run this command to map them from the top-level so they can be utilized. You can also configure a periodic task to have Nova discover new hosts automatically by setting the `scheduler.discover_hosts_in_cells_interval` to a time interval in seconds. The periodic task is run by the nova-scheduler service, so you must be sure to configure it on all of your nova-scheduler hosts.

Note

In the future, whenever you add new compute hosts, you will need to run the nova-manage cell_v2 discover_hosts command after starting them to map them to the cell if you did not configure automatic host discovery using `scheduler.discover_hosts_in_cells_interval`.

### Adding a new cell to an existing deployment

You can add additional cells to your deployment using the same steps used above to create your first cell. We can create a new cell record using nova-manage cell_v2 create_cell. For example:

    $ nova-manage cell_v2 create_cell \
        --name cell2 \
        --database_connection  mysql+pymysql://root:secretmysql@127.0.0.1/nova?charset=utf8 \
        --transport-url rabbit://stackrabbit:secretrabbit@mqserver:5672/

Note

If you don’t specify the database and transport urls then nova-manage will use the `transport_url` and `database.connection` values from the config file.

Note

It is a good idea to specify a name for the new cell you create so you can easily look up cell UUIDs with the nova-manage cell_v2 list_cells command later if needed.

Note

The nova-manage cell_v2 create_cell command will print the UUID of the newly-created cell if `--verbose` is passed, which is useful if you need to run commands like nova-manage cell_v2 discover_hosts targeted at a specific cell.

You can repeat this step for each cell you wish to add to your deployment. Your existing cell database will be reused - this simply informs the top-level API database about your existing cell databases.

Once you’ve created your new cell, use nova-manage cell_v2 discover_hosts to map compute hosts to cells. This is only necessary if you haven’t enabled automatic discovery using the `scheduler.discover_hosts_in_cells_interval` option. For example:

    $ nova-manage cell_v2 discover_hosts

Note

This command will search for compute hosts in each cell database and map them to the corresponding cell. This can be slow, particularly for larger deployments. You may wish to specify the `--cell_uuid` option, which will limit the search to a specific cell. You can use the nova-manage cell_v2 list_cells command to look up cell UUIDs if you are going to specify `--cell_uuid`.

Finally, run the nova-manage cell_v2 map_instances command to map existing instances to the new cell(s). For example:

    $ nova-manage cell_v2 map_instances

Note

This command will search for instances in each cell database and map them to the correct cell. This can be slow, particularly for larger deployments. You may wish to specify the `--cell_uuid` option, which will limit the search to a specific cell. You can use the nova-manage cell_v2 list_cells command to look up cell UUIDs if you are going to specify `--cell_uuid`.

Note

The `--max-count` option can be specified if you would like to limit the number of instances to map in a single run. If `--max-count` is not specified, all instances will be mapped. Repeated runs of the command will start from where the last run finished so it is not necessary to increase `--max-count` to finish. An exit code of 0 indicates that all instances have been mapped. An exit code of 1 indicates that there are remaining instances that need to be mapped.

### Template URLs in Cell Mappings

Starting in the 18.0.0 (Rocky) release, the URLs provided in the cell mappings for `--database_connection` and `--transport-url` can contain variables which are evaluated each time they are loaded from the database, and the values of which are taken from the corresponding base options in the host’s configuration file. The base URL is parsed and the following elements may be substituted into the cell mapping URL (using `rabbit://bob:s3kret@myhost:123/nova?sync=true#extra`):

Cell Mapping URL Variables
Variable | Meaning | Part of example URL
--- | --- | ---
`scheme` | The part before the `://` | `rabbit`
`username` | The username part of the credentials | `bob`
`password` | The password part of the credentials | `s3kret`
`hostname` | The hostname or address | `myhost`
`port` | The port number (must be specified) | `123`
`path` | The “path” part of the URL (without leading slash) | `nova`
`query` | The full query string arguments (without leading question mark) | `sync=true`
`fragment` | Everything after the first hash mark | `extra`

Variables are provided in curly brackets, like `{username}`. A simple template of `rabbit://{username}:{password}@otherhost/{path}` will generate a full URL of `rabbit://bob:s3kret@otherhost/nova` when used with the above example.

Note

The `database.connection` and `transport_url` values are not reloaded from the configuration file during a SIGHUP, which means that a full service restart will be required to notice changes in a cell mapping record if variables are changed.

Note

The `transport_url` option can contain an extended syntax for the “netloc” part of the URL (i.e. `userA:passwordA@hostA:portA,userB:passwordB@hostB:portB`). In this case, substitutions of the form `username1`, `username2`, etc will be honored and can be used in the template URL.

The templating of these URLs may be helpful in order to provide each service host with its own credentials for, say, the database. Without templating, all hosts will use the same URL (and thus credentials) for accessing services like the database and message queue. By using a URL with a template that results in the credentials being taken from the host-local configuration file, each host will use different values for those connections.

Assuming you have two service hosts that are normally configured with the cell0 database as their primary connection, their (abbreviated) configurations would look like this:

    [database]
    connection = mysql+pymysql://service1:foo@myapidbhost/nova_cell0

and:

    [database]
    connection = mysql+pymysql://service2:bar@myapidbhost/nova_cell0

Without cell mapping template URLs, they would still use the same credentials (as stored in the mapping) to connect to the cell databases. However, consider template URLs like the following:

    mysql+pymysql://{username}:{password}@mycell1dbhost/nova

and:

    mysql+pymysql://{username}:{password}@mycell2dbhost/nova

Using the first service and cell1 mapping, the calculated URL that will actually be used for connecting to that database will be:

    mysql+pymysql://service1:foo@mycell1dbhost/nova

## Design

Prior to the introduction of cells v2, when a request hit the Nova API for a particular instance, the instance information was fetched from the database. The information contained the hostname of the compute node on which the instance was currently located. If the request needed to take action on the instance (which it generally would), the hostname was used to calculate the name of a queue and a message was written there which would eventually find its way to the proper compute node.

The meat of the cells v2 feature was to split this hostname lookup into two parts that yielded three pieces of information instead of one. Basically, instead of merely looking up the name of the compute node on which an instance was located, we also started obtaining database and queue connection information. Thus, when asked to take action on instance $foo, we now:

1. Lookup the three-tuple of (database, queue, hostname) for that instance

2. Connect to that database and fetch the instance record

3. Connect to the queue and send the message to the proper hostname queue

The above differs from the previous organization in two ways. First, we now need to do two database lookups before we know where the instance lives. Second, we need to demand-connect to the appropriate database and queue. Both of these changes had performance implications, but it was possible to mitigate them through the use of things like a memcache of instance mapping information and pooling of connections to database and queue systems. The number of cells will always be much smaller than the number of instances.

There were also availability implications with the new feature since something like a instance list which might query multiple cells could end up with a partial result if there is a database failure in a cell. These issues can be mitigated, as discussed in Handling cell failures. A database failure within a cell would cause larger issues than a partial list result so the expectation is that it would be addressed quickly and cells v2 will handle it by indicating in the response that the data may not be complete.

## Comparison with cells v1

Prior to the introduction of cells v2, nova had a very similar feature, also called cells or referred to as cells v1 for disambiguation. Cells v2 was an effort to address many of the perceived shortcomings of the cell v1 feature. Benefits of the cells v2 feature over the previous cells v1 feature include:

- Native sharding of the database and queue as a first-class-feature in nova. All of the code paths will go through the lookup procedure and thus we won’t have the same feature parity issues as we do with current cells.

- No high-level replication of all the cell databases at the top. The API will need a database of its own for things like the instance index, but it will not need to replicate all the data at the top level.

- It draws a clear line between global and local data elements. Things like flavors and keypairs are clearly global concepts that need only live at the top level. Providing this separation allows compute nodes to become even more stateless and insulated from things like deleted/changed global data.

- Existing non-cells users will suddenly gain the ability to spawn a new “cell” from their existing deployment without changing their architecture. Simply adding information about the new database and queue systems to the new index will allow them to consume those resources.

- Existing cells users will need to fill out the cells mapping index, shutdown their existing cells synchronization service, and ultimately clean up their top level database. However, since the high-level organization is not substantially different, they will not have to re-architect their systems to move to cells v2.

- Adding new sets of hosts as a new “cell” allows them to be plugged into a deployment and tested before allowing builds to be scheduled to them.

## Caveats

Note

Many of these caveats have been addressed since the introduction of cells v2 in the 16.0.0 (Pike) release. These are called out below.

### Cross-cell move operations

Support for cross-cell cold migration and resize was introduced in the 21.0.0 (Ussuri) release. This is documented in Cross-cell resize. Prior to this release, it was not possible to cold migrate or resize an instance from a host in one cell to a host in another cell.

It is not currently possible to live migrate, evacuate or unshelve an instance from a host in one cell to a host in another cell.

### Quota-related quirks

Quotas are now calculated live at the point at which an operation would consume more resource, instead of being kept statically in the database. This means that a multi-cell environment may incorrectly calculate the usage of a tenant if one of the cells is unreachable, as those resources cannot be counted. In this case, the tenant may be able to consume more resource from one of the available cells, putting them far over quota when the unreachable cell returns.

Note

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.

Starting in the 2023.2 Bobcat (28.0.0) release, it is possible to configure unified limits quotas, which stores quota limits as Keystone unified limits and counts quota usage from the placement service and API database. See the unified limits documentation for more details.

### Performance of listing instances

Prior to the 17.0.0 (Queens) release, the instance list operation may not sort and paginate results properly when crossing multiple cell boundaries. Further, the performance of a sorted list operation across multiple cells was considerably slower than with a single cell. This was resolved as part of the efficient-multi-cell-instance-list-and-sort spec.

### Notifications

With a multi-cell environment with multiple message queues, it is likely that operators will want to configure a separate connection to a unified queue for notifications. This can be done in the configuration file of all nodes. Refer to the oslo.messaging configuration documentation for more details.

### Nova Metadata API service

Starting from the 19.0.0 (Stein) release, the nova metadata API service can be run either globally or per cell using the `api.local_metadata_per_cell` configuration option.

Global

If you have networks that span cells, you might need to run Nova metadata API globally by setting `api.local_metadata_per_cell` to `false`. When running globally, it should be configured as an API-level service with access to the `api_database.connection` information.

Local per cell

Running Nova metadata API per cell can have better performance and data isolation in a multi-cell deployment. If your networks are segmented along cell boundaries, then you can run Nova metadata API service per cell by setting `api.local_metadata_per_cell` to `true`. If you choose to run it per cell, you should also configure each neutron-metadata-agent service to point to the hostname/IP address of the corresponding Compute Metadata API.

### Console proxies

Starting from the 18.0.0 (Rocky) release, console proxies must be run per cell because console token authorizations are stored in cell databases. This means that each console proxy server must have access to the `database.connection` information for the cell database containing the instances for which it is proxying console access. This functionality was added as part of the convert-consoles-to-objects spec.

### Operations requiring upcalls

If you deploy multiple cells with a superconductor as described above, computes and cell-based conductors will not have the ability to speak to the scheduler as they are not connected to the same MQ. This is by design for isolation, but currently the processes are not in place to implement some features without such connectivity. Thus, anything that requires a so-called “upcall” will not function. This impacts the following:

1. Instance reschedules during boot and resize (part 1)

   Note

   This has been resolved in the Queens release.

2. Instance affinity reporting from the compute nodes to scheduler

3. The late anti-affinity check during server create and evacuate

4. Querying host aggregates from the cell

   Note

   This has been resolved in the Rocky release.

5. Attaching a volume and `[cinder] cross_az_attach = False`

6. Instance reschedules during boot and resize (part 2)

   Note

   This has been resolved in the Ussuri release.

The first is simple: if you boot an instance, it gets scheduled to a compute node, fails, it would normally be re-scheduled to another node. That requires scheduler intervention and thus it will not work in Pike with a multi-cell layout. If you do not rely on reschedules for covering up transient compute-node failures, then this will not affect you. To ensure you do not make futile attempts at rescheduling, you should set `scheduler.max_attempts` to `1` in `nova.conf`.

The second two are related. The summary is that some of the facilities that Nova has for ensuring that affinity/anti-affinity is preserved between instances does not function in Pike with a multi-cell layout. If you don’t use affinity operations, then this will not affect you. To make sure you don’t make futile attempts at the affinity check, you should set `workarounds.disable_group_policy_check_upcall` to `True` and `filter_scheduler.track_instance_changes` to `False` in `nova.conf`.

The fourth was previously only a problem when performing live migrations using the since-removed XenAPI driver and not specifying `--block-migrate`. The driver would attempt to figure out if block migration should be performed based on source and destination hosts being in the same aggregate. Since aggregates data had migrated to the API database, the cell conductor would not be able to access the aggregate information and would fail.

The fifth is a problem because when a volume is attached to an instance in the nova-compute service, and `[cinder]/cross_az_attach=False` in nova.conf, we attempt to look up the availability zone that the instance is in which includes getting any host aggregates that the `instance.host` is in. Since the aggregates are in the API database and the cell conductor cannot access that information, so this will fail. In the future this check could be moved to the nova-api service such that the availability zone between the instance and the volume is checked before we reach the cell, except in the case of boot from volume where the nova-compute service itself creates the volume and must tell Cinder in which availability zone to create the volume. Long-term, volume creation during boot from volume should be moved to the top-level superconductor which would eliminate this AZ up-call check problem.

The sixth is detailed in bug 1781286 and is similar to the first issue. The issue is that servers created without a specific availability zone will have their AZ calculated during a reschedule based on the alternate host selected. Determining the AZ for the alternate host requires an “up call” to the API DB.

## 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.

### Configuration considerations

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.

1. `database.max_retries` is 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.

2. `database.retry_interval` is 10 seconds and `oslo_messaging_rabbit.rabbit_retry_interval` is 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.

3. Nova also has a timeout value called `CELL_TIMEOUT` which 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.

### Known issues

1. 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.compute` is set to `auto` then the `nova-api` service 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.compute` to 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.

2. 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).

   Note

   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.

## FAQs

- How do I find out which hosts are bound to which cell?

  There are a couple of ways to do this.

  1. Run nova-manage cell_v2 discover_hosts --verbose.

     This does not produce a report but if you are trying to determine if a host is in a cell you can run this and it will report any hosts that are not yet mapped to a cell and map them. This command is idempotent.

  2. Run nova-manage cell_v2 list_hosts.

     This will list hosts in all cells. If you want to list hosts in a specific cell, you can use the `--cell_uuid` option.

- I updated the `database_connection` and/or `transport_url` in a cell using the `nova-manage cell_v2 update_cell` command but the API is still trying to use the old settings.

  The cell mappings are cached in the compute API service worker so you will need to restart the worker process to rebuild the cache. Note that there is another global cache tied to request contexts, which is used in the nova-conductor and nova-scheduler services, so you might need to do the same if you are having the same issue in those services. As of the 16.0.0 (Pike) release there is no timer on the cache or hook to refresh the cache using a SIGHUP to the service.

- I have upgraded from Newton to Ocata and I can list instances but I get a HTTP 404 (NotFound) error when I try to get details on a specific instance.

  Instances need to be mapped to cells so the API knows which cell an instance lives in. When upgrading, the nova-manage cell_v2 simple_cell_setup command will automatically map the instances to the single cell which is backed by the existing nova database. If you have already upgraded and did not use the nova-manage cell_v2 simple_cell_setup command, you can run the nova-manage cell_v2 map_instances command with the `--cell_uuid` option to map all instances in the given cell. See the Cells v2 Commands man page for details on command usage.

- Can I create a cell but have it disabled from scheduling?

  Yes. It is possible to create a pre-disabled cell such that it does not become a candidate for scheduling new VMs. This can be done by running the nova-manage cell_v2 create_cell command with the `--disabled` option.

- How can I disable a cell so that the new server create requests do not go to it while I perform maintenance?

  Existing cells can be disabled by running nova-manage cell_v2 update_cell with the `--disable` option and can be re-enabled once the maintenance period is over by running this command with the `--enable` option.

- I disabled (or enabled) a cell using the nova-manage cell_v2 update_cell or I created a new (pre-disabled) cell(mapping) using the nova-manage cell_v2 create_cell command but the scheduler is still using the old settings.

  The cell mappings are cached in the scheduler worker so you will either need to restart the scheduler process to refresh the cache, or send a SIGHUP signal to the scheduler by which it will automatically refresh the cells cache and the changes will take effect.

- Why was the cells REST API not implemented for cells v2? Why are there no CRUD operations for cells in the API?

  One of the deployment challenges that cells v1 had was the requirement for the API and control services to be up before a new cell could be deployed. This was not a problem for large-scale public clouds that never shut down, but is not a reasonable requirement for smaller clouds that do offline upgrades and/or clouds which could be taken completely offline by something like a power outage. Initial devstack and gate testing for cells v1 was delayed by the need to engineer a solution for bringing the services partially online in order to deploy the rest, and this continues to be a gap for other deployment tools. Consider also the FFU case where the control plane needs to be down for a multi-release upgrade window where changes to cell records have to be made. This would be quite a bit harder if the way those changes are made is via the API, which must remain down during the process.

  Further, there is a long-term goal to move cell configuration (i.e. cell_mappings and the associated URLs and credentials) into config and get away from the need to store and provision those things in the database. Obviously a CRUD interface in the API would prevent us from making that move.

- Why are cells not exposed as a grouping mechanism in the API for listing services, instances, and other resources?

  Early in the design of cells v2 we set a goal to not let the cell concept leak out of the API, even for operators. Aggregates are the way nova supports grouping of hosts for a variety of reasons, and aggregates can cut across cells, and/or be aligned with them if desired. If we were to support cells as another grouping mechanism, we would likely end up having to implement many of the same features for them as aggregates, such as scheduler features, metadata, and other searching/filtering operations. Since aggregates are how Nova supports grouping, we expect operators to use aggregates any time they need to refer to a cell as a group of hosts from the API, and leave actual cells as a purely architectural detail.

  The need to filter instances by cell in the API can and should be solved by adding a generic by-aggregate filter, which would allow listing instances on hosts contained within any aggregate, including one that matches the cell boundaries if so desired.

- Why are the API responses for `GET /servers`, `GET /servers/detail`, `GET /servers/{server_id}` and `GET /os-services` missing some information for certain cells at certain times? Why do I see the status as “UNKNOWN” for the servers in those cells at those times when I run `openstack server list` or `openstack server show`?

  Starting from microversion 2.69 the API responses of `GET /servers`, `GET /servers/detail`, `GET /servers/{server_id}` and `GET /os-services` may contain missing keys during down cell situations. See the Handling Down Cells section of the Compute API guide for more information on the partial constructs.

  For administrative considerations, see Handling cell failures.

## References

A large number of cells v2-related presentations have been given at various OpenStack and OpenInfra Summits over the years. These provide an excellent reference on the history and development of the feature along with details from real-world users of the feature.

- Newton Summit Video - Nova Cells V2: What’s Going On?

- Pike Summit Video - Scaling Nova: How CellsV2 Affects Your Deployment

- Queens Summit Video - Add Cellsv2 to your existing Nova deployment

- Rocky Summit Video - Moving from CellsV1 to CellsV2 at CERN

- Stein Summit Video - Scaling Nova with CellsV2: The Nova Developer and the CERN Operator perspective

- Train Summit Video - What’s new in Nova Cellsv2?

this page last updated: 2026-06-08 08:40:45

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