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3.13. Cells

Cells functionality enables you to scale an OpenStack Compute cloud in a more distributed fashion without having to use complicated technologies like database and message queue clustering. It supports very large deployments.
When this functionality is enabled, the hosts in an OpenStack Compute cloud are partitioned into groups called cells. Cells are configured as a tree. The top-level cell should have a host that runs a nova-api service, but no nova-compute services. Each child cell should run all of the typical nova-* services in a regular Compute cloud except for nova-api. You can think of cells as a normal Compute deployment in that each cell has its own database server and message queue broker.
The nova-cells service handles communication between cells and selects cells for new instances. This service is required for every cell. Communication between cells is pluggable, and currently the only option is communication through RPC.
Cells scheduling is separate from host scheduling. nova-cells first picks a cell. Once a cell is selected and the new build request reaches its nova-cells service, it is sent over to the host scheduler in that cell and the build proceeds as it would have without cells.
Warning
Cell functionality is currently considered experimental.

3.13.1. Cell configuration options
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Cells are disabled by default. All cell-related configuration options appear in the [cells] section in nova.conf. The following cell-related options are currently supported:
enable
Set to True to turn on cell functionality. Default is false.
name
Name of the current cell. Must be unique for each cell.
capabilities
List of arbitrary key=value pairs defining capabilities of the current cell. Values include hypervisor=xenserver;kvm,os=linux.
call_timeout
How long in seconds to wait for replies from calls between cells.
scheduler_filter_classes
Filter classes that the cells scheduler should use. By default, uses "nova.cells.filters.all_filters" to map to all cells filters included with Compute.
scheduler_weight_classes
Weight classes that the scheduler for cells uses. By default, uses nova.cells.weights.all_weighers to map to all cells weight algorithms included with Compute.
ram_weight_multiplier
Multiplier used to weight RAM. Negative numbers indicate that Compute should stack VMs on one host instead of spreading out new VMs to more hosts in the cell. The default value is 10.0.

3.13.2. Configure the API (top-level) cell
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The cell type must be changed in the API cell so that requests can be proxied through nova-cells down to the correct cell properly. Edit the nova.conf file in the API cell, and specify api in the cell_type key: 
[DEFAULT]
compute_api_class=nova.compute.cells_api.ComputeCellsAPI
...

[cells]
cell_type= api
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3.13.3. Configure the child cells
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Edit the nova.conf file in the child cells, and specify compute in the cell_type key: 
[DEFAULT]
# Disable quota checking in child cells. Let API cell do it exclusively.
quota_driver=nova.quota.NoopQuotaDriver

[cells]
cell_type = compute
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3.13.4. Configure the database in each cell
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Before bringing the services online, the database in each cell needs to be configured with information about related cells. In particular, the API cell needs to know about its immediate children, and the child cells must know about their immediate agents. The information needed is the RabbitMQ server credentials for the particular cell.
Use the nova-manage cell create command to add this information to the database in each cell: 
# nova-manage cell create -h
usage: nova-manage cell create [-h] [--name <name>]
                               [--cell_type <parent|api|child|compute>]
                               [--username <username>] [--password <password>]
                               [--broker_hosts <broker_hosts>]
                               [--hostname <hostname>] [--port <number>]
                               [--virtual_host <virtual_host>]
                               [--woffset <float>] [--wscale <float>]

optional arguments:
  -h, --help            show this help message and exit
  --name <name>         Name for the new cell
  --cell_type <parent|api|child|compute>
                        Whether the cell is parent/api or child/compute
  --username <username>
                        Username for the message broker in this cell
  --password <password>
                        Password for the message broker in this cell
  --broker_hosts <broker_hosts>
                        Comma separated list of message brokers in this cell.
                        Each Broker is specified as hostname:port with both
                        mandatory. This option overrides the --hostname and
                        --port options (if provided).
  --hostname <hostname>
                        Address of the message broker in this cell
  --port <number>       Port number of the message broker in this cell
  --virtual_host <virtual_host>
                        The virtual host of the message broker in this cell
  --woffset <float>
  --wscale <float>
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As an example, assume an API cell named api and a child cell named cell1.
Within the api cell, specify the following RabbitMQ server information: 
rabbit_host=10.0.0.10
rabbit_port=5672
rabbit_username=api_user
rabbit_password=api_passwd
rabbit_virtual_host=api_vhost
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Within the cell1 child cell, specify the following RabbitMQ server information: 
rabbit_host=10.0.1.10
rabbit_port=5673
rabbit_username=cell1_user
rabbit_password=cell1_passwd
rabbit_virtual_host=cell1_vhost
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You can run this in the API cell as root: 
# nova-manage cell create --name cell1 --cell_type child --username cell1_user --password cell1_passwd --hostname 10.0.1.10 --port 5673 --virtual_host cell1_vhost --woffset 1.0 --wscale 1.0
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Repeat the previous steps for all child cells.
In the child cell, run the following, as root: 
# nova-manage cell create --name api --cell_type parent --username api_user --password api_passwd --hostname 10.0.0.10 --port 5672 --virtual_host api_vhost --woffset 1.0 --wscale 1.0
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To customize the Compute cells, use the configuration option settings documented in Table 3.17, “Description of cell configuration options”.

3.13.5. Cell scheduling configuration
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To determine the best cell to use to launch a new instance, Compute uses a set of filters and weights defined in the /etc/nova/nova.conf file. The following options are available to prioritize cells for scheduling:
scheduler_filter_classes
List of filter classes. By default nova.cells.filters.all_filters is specified, which maps to all cells filters included with Compute (see Section 3.12.2, “Filters”).
scheduler_weight_classes
List of weight classes. By default nova.cells.weights.all_weighers is specified, which maps to all cell weight algorithms included with Compute. The following modules are available:
  • mute_child. Downgrades the likelihood of child cells being chosen for scheduling requests, which have not sent capacity or capability updates in a while. Options include mute_weight_multiplier (multiplier for mute children; value should be negative) and mute_weight_value (assigned to mute children; should be a positive value).
  • ram_by_instance_type. Select cells with the most RAM capacity for the instance type being requested. Because higher weights win, Compute returns the number of available units for the instance type requested. The ram_weight_multiplier option defaults to 10.0 that adds to the weight by a factor of 10. Use a negative number to stack VMs on one host instead of spreading out new VMs to more hosts in the cell.
  • weight_offset. Allows modifying the database to weight a particular cell. You can use this when you want to disable a cell (for example, '0'), or to set a default cell by making its weight_offset very high (for example, '999999999999999'). The highest weight will be the first cell to be scheduled for launching an instance.
Additionally, the following options are available for the cell scheduler:
scheduler_retries
Specifies how many times the scheduler tries to launch a new instance when no cells are available (default=10).
scheduler_retry_delay
Specifies the delay (in seconds) between retries (default=2).
As an admin user, you can also add a filter that directs builds to a particular cell. The policy.json file must have a line with "cells_scheduler_filter:TargetCellFilter" : "is_admin:True" to let an admin user specify a scheduler hint to direct a build to a particular cell.

3.13.6. Optional cell configuration
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Cells store all inter-cell communication data, including user names and passwords, in the database. Because the cells data is not updated very frequently, use the [cells]cells_config option to specify a JSON file to store cells data. With this configuration, the database is no longer consulted when reloading the cells data. The file must have columns present in the Cell model (excluding common database fields and the id column). You must specify the queue connection information through a transport_url field, instead of username, password, and so on. The transport_url has the following form: 
rabbit://USERNAME:PASSWORD@HOSTNAME:PORT/VIRTUAL_HOST
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The scheme can be either qpid or rabbit, as shown previously. The following sample shows this optional configuration: 
{
    "parent": {
        "name": "parent",
        "api_url": "http://api.example.com:8774",
        "transport_url": "rabbit://rabbit.example.com",
        "weight_offset": 0.0,
        "weight_scale": 1.0,
        "is_parent": true
    },
    "cell1": {
        "name": "cell1",
        "api_url": "http://api.example.com:8774",
        "transport_url": "rabbit://rabbit1.example.com",
        "weight_offset": 0.0,
        "weight_scale": 1.0,
        "is_parent": false
    },
    "cell2": {
        "name": "cell2",
        "api_url": "http://api.example.com:8774",
        "transport_url": "rabbit://rabbit2.example.com",
        "weight_offset": 0.0,
        "weight_scale": 1.0,
        "is_parent": false
    }
}
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