CellsCells 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.Cell functionality is currently considered
experimental.Cell configuration optionsCells 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:Set to True to turn
on cell functionality. Default is
false.Name of the current cell. Must be unique
for each cell.List of arbitrary
key=value
pairs defining capabilities of the current
cell. Values include
hypervisor=xenserver;kvm,os=linux;windows.How long in seconds to wait for replies
from calls between cells.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.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.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.Configure the API (top-level) cellThe compute API class must be changed in the API cell so
that requests can be proxied through nova-cells down to
the correct cell properly. Add the following line to
nova.conf in the API
cell:[DEFAULT]
compute_api_class=nova.compute.cells_api.ComputeCellsAPI
...
[cells]
enable=True
name=apiConfigure the child cellsAdd the following lines to
nova.conf in the child cells,
replacing cell1 with the name
of each
cell:[DEFAULT]
# Disable quota checking in child cells. Let API cell do it exclusively.
quota_driver=nova.quota.NoopQuotaDriver
[cells]
enable=True
name=cell1Configure the database in each cellBefore 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 -hOptions:
-h, --help show this help message and exit
--name=<name> Name for the new cell
--cell_type=<parent|child>
Whether the cell is a parent or child
--username=<username>
Username for the message broker in this cell
--password=<password>
Password for the message broker in this cell
--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>
(weight offset) It might be used by some cell scheduling code in the future
--wscale=<float>
(weight scale) It might be used by some cell scheduling code in the futureAs 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_vhostWithin 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_vhostYou 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=1.0Repeat 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=1.0To customize the Compute cells, use the configuration
option settings documented in .Cell scheduling configurationTo 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:List of filter classes. By
default
is specified, which maps to all cells filters
included with Compute (see ).List of weight classes. By
default
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 haven't sent capacity
or capability updates in a while.
Options include
(multiplier for mute children; value
should be negative) and
(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
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:Specifies how many times the scheduler tries
to launch a new instance when no cells are
available (default=10).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.Optional cell configurationCells 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
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
column). You must specify the
queue connection information through a
field, instead of
, ,
and so on. The has the
following form:rabbit://USERNAME:PASSWORD@HOSTNAME:PORT/VIRTUAL_HOSTThe scheme can be either qpid or
rabbit, as shown previously. The
following sample shows this optional configuration: