openstack/nova
Code Issues Proposed changes

docs: Add a new cells v2 document

Browse Source 
We currently have three cells v2 documents in-tree:

- A 'user/cellsv2-layout' document that details the structure or
  architecture of a cells v2 deployment (which is to say, any modern
  nova deployment)
- A 'user/cells' document, which is written from a pre-cells v2
  viewpoint and details the changes that cells v2 *will* require and the
  benefits it *would* bring. It also includes steps for upgrading from
  pre-cells v2 (that is, pre-Pike) deployment or a deployment with cells
  v1 (which we removed in Train and probably broke long before)
- An 'admin/cells' document, which doesn't contain much other than some
  advice for handling down cells

Clearly there's a lot of cruft to be cleared out as well as some
centralization of information that's possible. As such, we combine all
of these documents into one document, 'admin/cells'. This is chosen over
'users/cells' since cells are not an end-user-facing feature. References
to cells v1 and details on upgrading from pre-cells v2 deployments are
mostly dropped, as are some duplicated installation/configuration steps.
Formatting is fixed and Sphinx-isms used to cross reference config
option where possible. Finally, redirects are added so that people can
continue to find the relevant resources. The result is (hopefully) a
one stop shop for all things cells v2-related that operators can use to
configure and understand their deployments.

Change-Id: If39db50fd8b109a5a13dec70f8030f3663555065
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>
This commit is contained in:
Stephen Finucane 2021-07-09 17:58:52 +01:00
parent 0564006a0a
commit 13b06ebb1c
13 changed files with 1146 additions and 1233 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
doc/source/_extra/.htaccess
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@ -9,12 +9,12 @@ redirectmatch 301 ^/nova/([^/]+)/api_plugins.html$ /nova/$1/contributor/api.html
redirectmatch 301 ^/nova/([^/]+)/architecture.html$ /nova/$1/user/architecture.html
redirectmatch 301 ^/nova/([^/]+)/block_device_mapping.html$ /nova/$1/user/block-device-mapping.html
redirectmatch 301 ^/nova/([^/]+)/blueprints.html$ /nova/$1/contributor/blueprints.html
redirectmatch 301 ^/nova/([^/]+)/cells.html$ /nova/$1/user/cells.html
redirectmatch 301 ^/nova/([^/]+)/cells.html$ /nova/$1/admin/cells.html
redirectmatch 301 ^/nova/([^/]+)/code-review.html$ /nova/$1/contributor/code-review.html
redirectmatch 301 ^/nova/([^/]+)/conductor.html$ /nova/$1/user/conductor.html
redirectmatch 301 ^/nova/([^/]+)/development.environment.html$ /nova/$1/contributor/development-environment.html
redirectmatch 301 ^/nova/([^/]+)/devref/api.html /nova/$1/contributor/api.html
redirectmatch 301 ^/nova/([^/]+)/devref/cells.html /nova/$1/user/cells.html
redirectmatch 301 ^/nova/([^/]+)/devref/cells.html /nova/$1/admin/cells.html
redirectmatch 301 ^/nova/([^/]+)/devref/filter_scheduler.html /nova/$1/admin/scheduling.html
# catch all, if we hit something in devref assume it moved to
# reference unless we have already triggered a hit above.
@ -64,7 +64,9 @@ redirectmatch 301 ^/nova/([^/]+)/testing/zero-downtime-upgrade.html$ /nova/$1/co
redirectmatch 301 ^/nova/([^/]+)/threading.html$ /nova/$1/reference/threading.html
redirectmatch 301 ^/nova/([^/]+)/upgrade.html$ /nova/$1/admin/upgrades.html
redirectmatch 301 ^/nova/([^/]+)/user/aggregates.html$ /nova/$1/admin/aggregates.html
redirectmatch 301 ^/nova/([^/]+)/user/cellsv2_layout.html$ /nova/$1/user/cellsv2-layout.html
redirectmatch 301 ^/nova/([^/]+)/user/cells.html$ /nova/$1/admin/cells.html
redirectmatch 301 ^/nova/([^/]+)/user/cellsv2-layout.html$ /nova/$1/admin/cells.html
redirectmatch 301 ^/nova/([^/]+)/user/cellsv2_layout.html$ /nova/$1/admin/cells.html
redirectmatch 301 ^/nova/([^/]+)/user/config-drive.html$ /nova/$1/user/metadata.html
redirectmatch 301 ^/nova/([^/]+)/user/filter-scheduler.html$ /nova/$1/admin/scheduling.html
redirectmatch 301 ^/nova/([^/]+)/user/metadata-service.html$ /nova/$1/user/metadata.html

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61
doc/source/admin/configuration/cross-cell-resize.rst
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@ -2,21 +2,25 @@
Cross-cell resize
=================
This document describes how to configure nova for cross-cell resize.
For information on :term:`same-cell resize <Same-Cell Resize>`, refer to
:doc:`/admin/configuration/resize`.
.. note::
This document describes how to configure nova for cross-cell resize.
For information on :term:`same-cell resize <Same-Cell Resize>`, refer to
:doc:`/admin/configuration/resize`. For information on the cells v2 feature,
refer to :doc:`/admin/cells`.
Historically resizing and cold migrating a server has been explicitly
`restricted`_ to within the same cell in which the server already exists.
`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.
The full design details are in the `Ussuri spec`__ and there is a `video`__
from a summit talk with a high-level overview.
.. __: https://opendev.org/openstack/nova/src/tag/20.0.0/nova/conductor/tasks/migrate.py#L164
.. __: https://specs.openstack.org/openstack/nova-specs/specs/ussuri/approved/cross-cell-resize.html
.. __: https://www.openstack.org/videos/summits/denver-2019/whats-new-in-nova-cellsv2
.. _restricted: https://opendev.org/openstack/nova/src/tag/20.0.0/nova/conductor/tasks/migrate.py#L164
.. _Ussuri spec: https://specs.openstack.org/openstack/nova-specs/specs/ussuri/approved/cross-cell-resize.html
.. _video: https://www.openstack.org/videos/summits/denver-2019/whats-new-in-nova-cellsv2
Use case
--------
@ -32,6 +36,7 @@ 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.
Requirements
------------
@ -70,6 +75,7 @@ Networking
The networking API must expose the ``Port Bindings Extended`` API extension
which was added in the 13.0.0 (Rocky) release for Neutron.
Notifications
-------------
@ -78,13 +84,14 @@ 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
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.
.. _finish_revert_resize: https://opendev.org/openstack/nova/src/tag/20.0.0/nova/compute/manager.py#L4326
.. __: https://opendev.org/openstack/nova/src/tag/20.0.0/nova/compute/manager.py#L4326
Instance actions
----------------
@ -96,6 +103,7 @@ 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.
Scheduling
----------
@ -107,19 +115,20 @@ cell. However, this behavior is configurable using the
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.
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
`orchestrated`__ from the (super)conductor service. This uses the
:oslo.config:option:`long_rpc_timeout` configuration option.
* The orchestration tasks in the (super)conductor service are in charge of
@ -129,15 +138,16 @@ resize:
``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`_
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.
.. _confirmResize: https://docs.openstack.org/api-ref/compute/#confirm-resized-server-confirmresize-action
.. _revertResize: https://docs.openstack.org/api-ref/compute/#revert-resized-server-revertresize-action
.. _orchestrated: https://opendev.org/openstack/nova/src/branch/master/nova/conductor/tasks/cross_cell_migrate.py
.. _shelveOffload: https://docs.openstack.org/api-ref/compute/#shelf-offload-remove-server-shelveoffload-action
.. __: https://docs.openstack.org/api-ref/compute/#confirm-resized-server-confirmresize-action
.. __: https://docs.openstack.org/api-ref/compute/#revert-resized-server-revertresize-action
.. __: https://opendev.org/openstack/nova/src/branch/master/nova/conductor/tasks/cross_cell_migrate.py
.. __: https://docs.openstack.org/api-ref/compute/#shelf-offload-remove-server-shelveoffload-action
Sequence diagram
----------------
@ -230,6 +240,7 @@ status.
}
Limitations
-----------
@ -251,19 +262,21 @@ 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
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
:oslo.config:option:`configured <resize_confirm_window>` to automatically
confirm pending resizes on the target host, *might* not support cross-cell
resizes because doing so would require an :ref:`up-call <upcall>` to the
API to confirm the resize and cleanup the source cell database.
.. _personality files: https://docs.openstack.org/api-guide/compute/server_concepts.html#server-personality
.. _evacuating: https://docs.openstack.org/api-ref/compute/#evacuate-server-evacuate-action
.. __: https://docs.openstack.org/api-guide/compute/server_concepts.html#server-personality
.. __: https://docs.openstack.org/api-ref/compute/#evacuate-server-evacuate-action
Troubleshooting
---------------
@ -301,9 +314,9 @@ 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
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.
.. _rebuild: https://docs.openstack.org/api-ref/compute/#rebuild-server-rebuild-action
.. __: https://docs.openstack.org/api-ref/compute/#rebuild-server-rebuild-action

3
doc/source/admin/configuring-migrations.rst
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@ -63,7 +63,8 @@ The migration types are:
.. note::
In a multi-cell cloud, instances can be live migrated to a
different host in the same cell, but not across cells.
different host in the same cell, but not across cells. Refer to the
:ref:`cells v2 documentation <cells-v2-caveats>`. for more information.
The following sections describe how to configure your hosts for live migrations
using the libvirt virt driver and KVM hypervisor.

9
doc/source/admin/troubleshooting/affinity-policy-violated.rst
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@ -43,9 +43,9 @@ reschedule of the server create request. Note that this means the deployment
must have :oslo.config:option:`scheduler.max_attempts` set greater than ``1``
(default is ``3``) to handle races.
An ideal configuration for multiple cells will minimize `upcalls`_ from the
cells to the API database. This is how devstack, for example, is configured in
the CI gate. The cell conductors do not set
An ideal configuration for multiple cells will minimize :ref:`upcalls <upcall>`
from the cells to the API database. This is how devstack, for example, is
configured in the CI gate. The cell conductors do not set
:oslo.config:option:`api_database.connection` and ``nova-compute`` sets
:oslo.config:option:`workarounds.disable_group_policy_check_upcall` to
``True``.
@ -73,6 +73,3 @@ parallel server create requests are racing.
Future work is needed to add anti-/affinity support to the placement service in
order to eliminate the need for the late affinity check in ``nova-compute``.
.. _upcalls: https://docs.openstack.org/nova/latest/user/cellsv2-layout.html#operations-requiring-upcalls

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doc/source/admin/upgrades.rst
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@ -53,8 +53,8 @@ same time.
deployment. The `nova-conductor`` service will fail to start when a
``nova-compute`` service that is older than the previous release (N-2 or
greater) is detected. Similarly, in a :doc:`deployment with multiple cells
</user/cellsv2-layout>`, neither the super conductor service nor any
per-cell conductor service will start if any other conductor service in the
</admin/cells>`, neither the super conductor service nor any per-cell
conductor service will start if any other conductor service in the
deployment is older than the previous release.
#. Before maintenance window:

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doc/source/index.rst
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@ -153,9 +153,10 @@ the defaults from the :doc:`install guide </install/index>` will be sufficient.
* :doc:`Feature Support full list </user/support-matrix>`: A detailed dive through
features in each compute driver backend.
* :doc:`Cells v2 Planning </user/cellsv2-layout>`: For large deployments, Cells v2
allows sharding of your compute environment. Upfront planning is key to a
successful Cells v2 layout.
* :doc:`Cells v2 configuration </admin/cells>`: For large deployments, cells v2
cells allow sharding of your compute environment. Upfront planning is key to
a successful cells v2 layout.
* :doc:`Running nova-api on wsgi <user/wsgi>`: Considerations for using a real
WSGI container instead of the baked-in eventlet web server.
@ -166,7 +167,7 @@ the defaults from the :doc:`install guide </install/index>` will be sufficient.
user/feature-classification
user/support-matrix
user/cellsv2-layout
admin/cells
user/wsgi
Maintenance

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@ -92,10 +92,9 @@ these documents will move into the "Internals" section.
If you want to get involved in shaping the future of nova's architecture,
these are a great place to start reading up on the current plans.
* :doc:`/user/cells`: How cells v2 is evolving
* :doc:`/reference/policy-enforcement`: How we want policy checks on API actions
to work in the future
* :doc:`/reference/stable-api`: What stable api means to nova
* :doc:`/reference/stable-api`: What stable API means to nova
* :doc:`/reference/scheduler-evolution`: Motivation behind the scheduler /
placement evolution
@ -104,7 +103,6 @@ these are a great place to start reading up on the current plans.
.. toctree::
:hidden:
/user/cells
policy-enforcement
stable-api
scheduler-evolution

2
doc/source/user/architecture.rst
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@ -42,7 +42,7 @@ To make this possible nova-compute proxies DB requests over RPC to a
central manager called ``nova-conductor``.
To horizontally expand Nova deployments, we have a deployment sharding
concept called cells. For more information please see: :doc:`cells`
concept called cells. For more information please see: :doc:`/admin/cells`
Components
----------

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doc/source/user/cells.rst
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@ -1,759 +0,0 @@
..
Licensed under the Apache License, Version 2.0 (the "License"); you may
not use this file except in compliance with the License. You may obtain
a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
.. _cells:
=======
Cells
=======
Before reading further, there is a nice overview presentation_ that
Andrew Laski gave at the Austin (Newton) summit which is worth watching.
.. _presentation: https://www.openstack.org/videos/summits/austin-2016/nova-cells-v2-whats-going-on
.. _cells-v2:
Cells V2
========
* `Newton Summit Video - Nova Cells V2: What's Going On? <https://www.openstack.org/videos/austin-2016/nova-cells-v2-whats-going-on>`_
* `Pike Summit Video - Scaling Nova: How CellsV2 Affects Your Deployment <https://www.openstack.org/videos/boston-2017/scaling-nova-how-cellsv2-affects-your-deployment>`_
* `Queens Summit Video - Add Cellsv2 to your existing Nova deployment <https://www.openstack.org/videos/sydney-2017/adding-cellsv2-to-your-existing-nova-deployment>`_
* `Rocky Summit Video - Moving from CellsV1 to CellsV2 at CERN
<https://www.openstack.org/videos/summits/vancouver-2018/moving-from-cellsv1-to-cellsv2-at-cern>`_
* `Stein Summit Video - Scaling Nova with CellsV2: The Nova Developer and the CERN Operator perspective
<https://www.openstack.org/videos/summits/berlin-2018/scaling-nova-with-cellsv2-the-nova-developer-and-the-cern-operator-perspective>`_
Manifesto
~~~~~~~~~
Proposal
--------
Right now, when a request hits the Nova API for a particular instance, the
instance information is fetched from the database, which contains the hostname
of the compute node on which the instance currently lives. If the request needs
to take action on the instance (which is most of them), the hostname is used to
calculate the name of a queue, and a message is written there which finds its
way to the proper compute node.
The meat of this proposal is changing the above hostname lookup into two parts
that yield three pieces of information instead of one. Basically, instead of
merely looking up the *name* of the compute node on which an instance lives, we
will also obtain database and queue connection information. Thus, when asked to
take action on instance $foo, we will:
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 current organization in two ways. First, we 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
have performance implications, but we believe we can mitigate the impacts
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 are availability implications with this change since something like a
'nova list' which might query multiple cells could end up with a partial result
if there is a database failure in a cell. See :doc:`/admin/cells` for knowing
more about the recommended practices under such situations. 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 cellsv2 will handle it by
indicating in the response that the data may not be complete.
Since this is very similar to what we have with current cells, in terms of
organization of resources, we have decided to call this "cellsv2" for
disambiguation.
After this work is complete there will no longer be a "no cells" deployment.
The default installation of Nova will be a single cell setup.
Benefits
--------
The benefits of this new organization are:
* 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 cellsv2.
* 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.
Database split
~~~~~~~~~~~~~~
As mentioned above there is a split between global data and data that is local
to a cell.
The following is a breakdown of what data can uncontroversially considered
global versus local to a cell. Missing data will be filled in as consensus is
reached on the data that is more difficult to cleanly place. The missing data
is mostly concerned with scheduling and networking.
Global (API-level) Tables
-------------------------
instance_types
instance_type_projects
instance_type_extra_specs
quotas
project_user_quotas
quota_classes
quota_usages
security_groups
security_group_rules
security_group_default_rules
provider_fw_rules
key_pairs
migrations
networks
tags
Cell-level Tables
-----------------
instances
instance_info_caches
instance_extra
instance_metadata
instance_system_metadata
instance_faults
instance_actions
instance_actions_events
instance_id_mappings
pci_devices
block_device_mapping
virtual_interfaces
Setup of Cells V2
=================
Overview
~~~~~~~~
As more of the CellsV2 implementation is finished, all operators are
required to make changes to their deployment. For all deployments
(even those that only intend to have one cell), 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.
Please only restart the services once you are done with the described
steps below.
First Time Setup
~~~~~~~~~~~~~~~~
Since there is only one API database, the connection information for
it is stored in the nova.conf file.
::
[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, you must have connection information
in your config file for the API database before continuing to the
steps below, so that `nova-manage` can find your other databases.
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.
The commands below use the API database so remember to run
`nova-manage api_db sync` first.
First we will create the necessary records for the cell0 database. To
do that we use `nova-manage` like this::
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
`nova-manage` 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 and then run
`map_cell0` again with the proper database connection value.
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 like this::
nova-manage cell_v2 create_cell --verbose --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
`[database]/connection` and `[DEFAULT]/transport_url` values
from the config file.
.. note:: 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.
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 `discover_hosts` targeted at a specific
cell.
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. Once that has
happened, you can scan and add it to the cell::
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.
Template URLs in Cell Mappings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Starting in the 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``):
.. list-table:: Cell Mapping URL Variables
:header-rows: 1
:widths: 15, 50, 15
* - 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
``[DEFAULT]/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 ``[DEFAULT]/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, substitions 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
References
~~~~~~~~~~
* :doc:`nova-manage man page </cli/nova-manage>`
Step-By-Step for Common Use Cases
=================================
The following are step-by-step examples for common use cases setting
up Cells V2. This is intended as a quick reference that puts together
everything explained in `Setup of Cells V2`_. It is assumed that you have
followed all other install steps for Nova and are setting up Cells V2
specifically at this point.
Fresh Install
~~~~~~~~~~~~~
You are installing Nova for the first time and have no compute hosts in the
database yet. This will set up a single cell Nova deployment.
1. Reminder: You should have already created and synced the Nova API database
by creating a database, configuring its connection in the
``[api_database]/connection`` setting in the Nova configuration file, and
running ``nova-manage api_db sync``.
2. Create a database for cell0. If you are going to pass the database
connection url on the command line in step 3, you can name the cell0
database whatever you want. If you are not going to pass the database url on
the command line in step 3, you need to name the cell0 database based on the
name of your existing Nova database: <Nova database name>_cell0. For
example, if your Nova database is named ``nova``, then your cell0 database
should be named ``nova_cell0``.
3. Run the ``map_cell0`` command to create and map cell0::
nova-manage cell_v2 map_cell0 \
--database_connection <database connection url>
The database connection url is generated based on the
``[database]/connection`` setting in the Nova configuration file if not
specified on the command line.
4. Run ``nova-manage db sync`` to populate the cell0 database with a schema.
The ``db sync`` command reads the database connection for cell0 that was
created in step 3.
5. Run the ``create_cell`` command to create the single cell which will contain
your compute hosts::
nova-manage cell_v2 create_cell --name <name> \
--transport-url <transport url for message queue> \
--database_connection <database connection url>
The transport url is taken from the ``[DEFAULT]/transport_url`` setting in
the Nova configuration file if not specified on the command line. The
database url is taken from the ``[database]/connection`` setting in the Nova
configuration file if not specified on the command line.
6. Configure your compute host(s), making sure ``[DEFAULT]/transport_url``
matches the transport URL for the cell created in step 5, and start the
nova-compute service. Before step 7, make sure you have compute hosts in the
database by running::
nova service-list --binary nova-compute
7. Run the ``discover_hosts`` command to map compute hosts to the single cell
by running::
nova-manage cell_v2 discover_hosts
The command will search for compute hosts in the database of the cell you
created in step 5 and map them to the cell. 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:: Remember: In the future, whenever you add new compute hosts, you
will need to run the ``discover_hosts`` command after starting them
to map them to the cell if you did not configure the automatic host
discovery in step 7.
Upgrade (minimal)
~~~~~~~~~~~~~~~~~
You are upgrading an existing Nova install and have compute hosts in the
database. This will set up a single cell Nova deployment.
1. If you haven't already created a cell0 database in a prior release,
create a database for cell0 with a name based on the name of your Nova
database: <Nova database name>_cell0. If your Nova database is named
``nova``, then your cell0 database should be named ``nova_cell0``.
.. warning:: In Newton, the ``simple_cell_setup`` command expects the name of
the cell0 database to be based on the name of the Nova API
database: <Nova API database name>_cell0 and the database
connection url is taken from the ``[api_database]/connection``
setting in the Nova configuration file.
2. Run the ``simple_cell_setup`` command to create and map cell0, create and
map the single cell, and map existing compute hosts and instances to the
single cell::
nova-manage cell_v2 simple_cell_setup \
--transport-url <transport url for message queue>
The transport url is taken from the ``[DEFAULT]/transport_url`` setting in
the Nova configuration file if not specified on the command line. The
database connection url will be generated based on the
``[database]/connection`` setting in the Nova configuration file.
.. note:: Remember: In the future, whenever you add new compute hosts, you
will need to run the ``discover_hosts`` command after starting them
to map them to the cell. 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.
Upgrade with Cells V1
~~~~~~~~~~~~~~~~~~~~~
.. todo:: This needs to be removed but `Adding a new cell to an existing deployment`_
is still using it.
You are upgrading an existing Nova install that has Cells V1 enabled and have
compute hosts in your databases. This will set up a multiple cell Nova
deployment. At this time, it is recommended to keep Cells V1 enabled during and
after the upgrade as multiple Cells V2 cell support is not fully finished and
may not work properly in all scenarios. These upgrade steps will help ensure a
simple cutover from Cells V1 to Cells V2 in the future.
.. note:: There is a Rocky summit video from CERN about how they did their
upgrade from cells v1 to v2 here:
https://www.openstack.org/videos/vancouver-2018/moving-from-cellsv1-to-cellsv2-at-cern
1. If you haven't already created a cell0 database in a prior release,
create a database for cell0. If you are going to pass the database
connection url on the command line in step 2, you can name the cell0
database whatever you want. If you are not going to pass the database url on
the command line in step 2, you need to name the cell0 database based on the
name of your existing Nova database: <Nova database name>_cell0. For
example, if your Nova database is named ``nova``, then your cell0 database
should be named ``nova_cell0``.
2. Run the ``map_cell0`` command to create and map cell0::
nova-manage cell_v2 map_cell0 \
--database_connection <database connection url>
The database connection url is generated based on the
``[database]/connection`` setting in the Nova configuration file if not
specified on the command line.
3. Run ``nova-manage db sync`` to populate the cell0 database with a schema.
The ``db sync`` command reads the database connection for cell0 that was
created in step 2.
4. Run the ``create_cell`` command to create cells which will contain your
compute hosts::
nova-manage cell_v2 create_cell --name <cell name> \
--transport-url <transport url for message queue> \
--database_connection <database connection url>
You will need to repeat this step for each cell in your deployment. Your
existing cell database will be re-used -- this simply informs the top-level
API database about your existing cell databases.
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 ``list_cells`` command later if needed.
The transport url is taken from the ``[DEFAULT]/transport_url`` setting in
the Nova configuration file if not specified on the command line. The
database url is taken from the ``[database]/connection`` setting in the Nova
configuration file if not specified on the command line. If you are not
going to specify ``--database_connection`` and ``--transport-url`` on the
command line, be sure to specify your existing cell Nova configuration
file::
nova-manage --config-file <cell nova.conf> cell_v2 create_cell \
--name <cell name>
5. Run the ``discover_hosts`` command to map compute hosts to cells::
nova-manage cell_v2 discover_hosts --cell_uuid <cell uuid>
You will need to repeat this step for each cell in your deployment unless
you omit the ``--cell_uuid`` option. If the cell uuid is not specified on
the command line, ``discover_hosts`` will search for compute hosts in each
cell database and map them to the corresponding cell. You can use the
``list_cells`` command to look up cell uuids if you are going to specify
``--cell_uuid``.
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.
6. Run the ``map_instances`` command to map instances to cells::
nova-manage cell_v2 map_instances --cell_uuid <cell uuid> \
--max-count <max count>
You will need to repeat this step for each cell in your deployment. You can
use the ``list_cells`` command to look up cell uuids.
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.
.. note:: Remember: In the future, whenever you add new compute hosts, you
will need to run the ``discover_hosts`` command after starting them
to map them to a cell if you did not configure the automatic host
discovery in step 5.
Adding a new cell to an existing deployment
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To expand your deployment with a new cell, first follow the usual steps for
standing up a new Cells V1 cell. After that is finished, follow step 4 in
`Upgrade with Cells V1`_ to create a new Cells V2 cell for it. If you have
added new compute hosts for the new cell, you will also need to run the
``discover_hosts`` command after starting them to map them to the new cell if
you did not configure the automatic host discovery as described in step 5 in
`Upgrade with Cells V1`_.
References
~~~~~~~~~~
* :doc:`nova-manage man page </cli/nova-manage>`
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 --config-file <cell config> host list``. This will
only lists hosts in the provided cell nova.conf.
.. deprecated:: 16.0.0
The ``nova-manage host list`` command is deprecated as of the
16.0.0 Pike release.
2. 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.
3. 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 run
``nova-manage cell_v2 list_hosts --cell_uuid <cell_uuid>``.
#. 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 nova-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
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 ``simple_cell_setup`` command, you can run the
``nova-manage cell_v2 map_instances --cell_uuid <cell_uuid>`` command to
map all instances in the given cell. See the :ref:`man-page-cells-v2` man
page for details on command usage.
#. Should I change any of the ``[cells]`` configuration options for Cells v2?
**NO**. Those options are for Cells v1 usage only and are not used at all
for Cells v2. That includes the ``nova-cells`` service - it has nothing
to do with Cells v2.
#. 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
--cell_uuid <cell_uuid> --disable`` and can be re-enabled once the
maintenance period is over by running ``nova-manage cell_v2 update_cell
--cell_uuid <cell_uuid> --enable``
#. 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 CellsV2? Why are
there no CRUD operations for cells in the API?
One of the deployment challenges that CellsV1 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 CellsV1 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 CellsV2 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
<https://docs.openstack.org/api-guide/compute/down_cells.html>`__
section of the Compute API guide for more information on the partial
constructs.
For administrative considerations, see
:ref:`Handling cell failures <handling-cell-failures>`.

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@ -1,412 +0,0 @@
..
Licensed under the Apache License, Version 2.0 (the "License"); you may
not use this file except in compliance with the License. You may obtain
a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
===================
Cells Layout (v2)
===================
This document describes the layout of a deployment with Cells
version 2, including deployment considerations for security and
scale. 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 cellsv2 support in Nova means that it
applies in some way to all deployments.
Concepts
========
A basic Nova system consists of the following components:
* The nova-api service 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 nova-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 service, as well as 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.
* A message queue which allows the services to communicate with each
other via RPC.
All deployments have at least the above components. Small deployments
likely have 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. This is considered a
"single-cell deployment" because it only has one "real" cell. The
cell0 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).
The purpose of the cells functionality in nova is specifically to
allow larger deployments to shard their many compute nodes into cells,
each of which has a database and message queue. The API database is
always and only global, but there can be many cell databases (where
the bulk of the instance information lives), each with a portion of
the instances for the entire deployment within.
All of the nova services use a configuration file, all of which will
at a minimum specify a message queue endpoint
(i.e. ``[DEFAULT]/transport_url``). Most of the services also require
configuration of database connection information
(i.e. ``[database]/connection``). 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 elements of the lower-level services. See the
``nova-manage`` :ref:`man-page-cells-v2` commands for more
information about how to create and examine these records.
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.
Simple
------
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:
.. graphviz::
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.
.. graphviz::
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 the
release notes for the version of Nova you are using for the most
up-to-date information about any caveats that may be present due to
this limitation.
Caveats of a Multi-Cell deployment
----------------------------------
.. note:: This information is correct as of the Pike release. Where
improvements have been made or issues fixed, they are noted per
item.
Cross-cell instance migrations
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Currently it is not possible to migrate an instance from a host in one
cell to a host in another cell. This may be possible in the future,
but it is currently unsupported. This impacts cold migration,
resizes, live migrations, evacuate, and unshelve operations.
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
:doc:`quotas documentation<quotas>` for more details.
Performance of listing instances
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. note:: This has been resolved in the Queens release [#]_.
With multiple cells, the instance list operation may not sort and
paginate results properly when crossing multiple cell
boundaries. Further, the performance of a sorted list operation will
be considerably slower than with a single cell.
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-doc:`oslo.messaging configuration
documentation
<configuration/opts.html#oslo_messaging_notifications.transport_url>` for more
details.
.. _cells-v2-layout-metadata-api:
Nova Metadata API service
~~~~~~~~~~~~~~~~~~~~~~~~~
Starting from the Stein release, the :doc:`nova metadata API service
</admin/metadata-service>` can be run either globally or per cell using the
:oslo.config:option:`api.local_metadata_per_cell` configuration option.
**Global**
If you have networks that span cells, you might need to run Nova metadata API
globally. When running globally, it should be configured as an API-level
service with access to the :oslo.config:option:`api_database.connection`
information. The nova metadata API service **must not** be run as a standalone
service, using the :program:`nova-api-metadata` service, in this case.
**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. If you
choose to run it per cell, you should also configure each
:neutron-doc:`neutron-metadata-agent
<configuration/metadata-agent.html?#DEFAULT.nova_metadata_host>` service to
point to the corresponding :program:`nova-api-metadata`. The nova metadata API
service **must** be run as a standalone service, using the
:program:`nova-api-metadata` service, in this case.
Console proxies
~~~~~~~~~~~~~~~
`Starting from the 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
:oslo.config:option:`database.connection` information for the cell database
containing the instances for which it is proxying console access.
.. __: https://specs.openstack.org/openstack/nova-specs/specs/rocky/implemented/convert-consoles-to-objects.html
.. _upcall:
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:
#. Instance reschedules during boot and resize (part 1)
.. note:: This has been resolved in the Queens release [#]_.
#. Instance affinity reporting from the compute nodes to scheduler
#. The late anti-affinity check during server create and evacuate
#. Querying host aggregates from the cell
.. note:: This has been resolved in the Rocky release [#]_.
#. Attaching a volume and ``[cinder]/cross_az_attach=False``
#. 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=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=True`` and
``[filter_scheduler]/track_instance_changes=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 :term:`boot from volume <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 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.
.. _bug 1781286: https://bugs.launchpad.net/nova/+bug/1781286
.. [#] https://blueprints.launchpad.net/nova/+spec/efficient-multi-cell-instance-list-and-sort
.. [#] https://specs.openstack.org/openstack/nova-specs/specs/queens/approved/return-alternate-hosts.html
.. [#] https://blueprints.launchpad.net/nova/+spec/live-migration-in-xapi-pool
.. [#] https://review.opendev.org/686047/
.. [#] https://review.opendev.org/686050/

6
doc/source/user/index.rst
View File

@ -54,9 +54,9 @@ the defaults from the :doc:`install guide </install/index>` will be sufficient.
* :doc:`Feature Support full list </user/support-matrix>`: A detailed dive through
features in each compute driver backend.
* :doc:`Cells v2 Planning </user/cellsv2-layout>`: For large deployments, Cells v2
allows sharding of your compute environment. Upfront planning is key to a
successful Cells v2 layout.
* :doc:`Cells v2 configuration </admin/cells>`: For large deployments, cells v2
cells allow sharding of your compute environment. Upfront planning is key to
a successful cells v2 layout.
* :placement-doc:`Placement service <>`: Overview of the placement
service, including how it fits in with the rest of nova.

8
doc/test/redirect-tests.txt
View File

@ -9,12 +9,12 @@
/nova/latest/architecture.html 301 /nova/latest/user/architecture.html
/nova/latest/block_device_mapping.html 301 /nova/latest/user/block-device-mapping.html
/nova/latest/blueprints.html 301 /nova/latest/contributor/blueprints.html
/nova/latest/cells.html 301 /nova/latest/user/cells.html
/nova/latest/cells.html 301 /nova/latest/admin/cells.html
/nova/latest/code-review.html 301 /nova/latest/contributor/code-review.html
/nova/latest/conductor.html 301 /nova/latest/user/conductor.html
/nova/latest/development.environment.html 301 /nova/latest/contributor/development-environment.html
/nova/latest/devref/api.html 301 /nova/latest/contributor/api.html
/nova/latest/devref/cells.html 301 /nova/latest/user/cells.html
/nova/latest/devref/cells.html 301 /nova/latest/admin/cells.html
/nova/latest/devref/filter_scheduler.html 301 /nova/latest/admin/scheduling.html
# catch all, if we hit something in devref assume it moved to
# reference unless we have already triggered a hit above.
@ -64,7 +64,9 @@
/nova/latest/threading.html 301 /nova/latest/reference/threading.html
/nova/latest/upgrade.html 301 /nova/latest/admin/upgrades.html
/nova/latest/user/aggregates.html 301 /nova/latest/admin/aggregates.html
/nova/latest/user/cellsv2_layout.html 301 /nova/latest/user/cellsv2-layout.html
/nova/latest/user/cells.html 301 /nova/latest/admin/cells.html
/nova/latest/user/cellsv2_layout.html 301 /nova/latest/admin/cells.html
/nova/latest/user/cellsv2-layout.html 301 /nova/latest/admin/cells.html
/nova/latest/user/config-drive.html 301 /nova/latest/user/metadata.html
/nova/latest/user/filter-scheduler.html 301 /nova/latest/admin/scheduling.html
/nova/latest/user/metadata-service.html 301 /nova/latest/user/metadata.html