Availability Zones are an end-user visible logical abstraction for
partitioning a cloud without knowing the physical infrastructure. They
can be used to partition a cloud on arbitrary factors, such as location
(country, datacenter, rack), network layout and/or power source.

Availability zones are not modeled in the database; rather, they are
defined by attaching specific metadata information to an aggregate </admin/aggregates> The addition of
this specific metadata to an aggregate makes the aggregate visible from
an end-user perspective and consequently allows users to schedule
instances to a specific set of hosts, the ones belonging to the
aggregate. There are a few additional differences to note when comparing
availability zones and host aggregates:

• A host can be part of multiple aggregates but it can only be in
  one availability zone.

• By default a host is part of a default availability zone even if
  it doesn’t belong to an aggregate. The name of this default availability
  zone can be configured using default_availability_zone config
  option.

  Warning

  The use of the default availability zone name in requests can be very
  error-prone. Since the user can see the list of availability zones, they
  have no way to know whether the default availability zone name
  (currently nova) is provided because an host belongs to an
  aggregate whose AZ metadata key is set to nova, or because
  there is at least one host not belonging to any aggregate. Consequently,
  it is highly recommended for users to never ever ask for booting an
  instance by specifying an explicit AZ named nova and for
  operators to never set the AZ metadata for an aggregate to
  nova. This can result is some problems due to the fact that
  the instance AZ information is explicitly attached to nova
  which could break further move operations when either the host is moved
  to another aggregate or when the user would like to migrate the
  instance.

  Note

  Availability zone names must NOT contain : since it is
  used by admin users to specify hosts where instances are launched in
  server creation. See Using availability
  zones to select hosts for more information.

In addition, other services, such as the networking service <> and the block storage service <>, also provide an
availability zone feature. However, the implementation of these features
differs vastly between these different services. Consult the
documentation for these other services for more information on their
implementation of this feature.

Availability Zones with
Placement

In order to use placement to honor availability zone requests, there
must be placement aggregates that match the membership and UUID of nova
host aggregates that you assign as availability zones. The same key in
aggregate metadata used by the AvailabilityZoneFilter filter controls this
function, and is enabled by setting scheduler.query_placement_for_availability_zone
to True. As of 24.0.0 (Xena), this is the default.

$ openstack --os-compute-api-version=2.53 aggregate create myaz
+-------------------+--------------------------------------+
| Field             | Value                                |
+-------------------+--------------------------------------+
| availability_zone | None                                 |
| created_at        | 2018-03-29T16:22:23.175884           |
| deleted           | False                                |
| deleted_at        | None                                 |
| id                | 4                                    |
| name              | myaz                                 |
| updated_at        | None                                 |
| uuid              | 019e2189-31b3-49e1-aff2-b220ebd91c24 |
+-------------------+--------------------------------------+

$ openstack --os-compute-api-version=2.53 aggregate add host myaz node1
+-------------------+--------------------------------------+
| Field             | Value                                |
+-------------------+--------------------------------------+
| availability_zone | None                                 |
| created_at        | 2018-03-29T16:22:23.175884           |
| deleted           | False                                |
| deleted_at        | None                                 |
| hosts             | [u'node1']                           |
| id                | 4                                    |
| name              | myagg                                |
| updated_at        | None                                 |
| uuid              | 019e2189-31b3-49e1-aff2-b220ebd91c24 |
+-------------------+--------------------------------------+

$ openstack aggregate set --property availability_zone=az002 myaz

$ openstack --os-placement-api-version=1.2 resource provider aggregate set --aggregate 019e2189-31b3-49e1-aff2-b220ebd91c24 815a5634-86fb-4e1e-8824-8a631fee3e06

Without the above configuration, the AvailabilityZoneFilter filter must be enabled
in filter_scheduler.enabled_filters to
retain proper behavior.

Implications for moving
servers

There are several ways to move a server to another host: evacuate,
resize, cold migrate, live migrate, and unshelve. Move operations
typically go through the scheduler to pick the target host
unless a target host is specified and the request forces the
server to that host by bypassing the scheduler. Only evacuate and live
migrate can forcefully bypass the scheduler and move a server to a
specified host and even then it is highly recommended to not
force and bypass the scheduler.

With respect to availability zones, a server is restricted to a zone
if:

1. The server was created in a specific zone with the
   POST /servers request containing the
   availability_zone parameter.
2. If the server create request did not contain the
   availability_zone parameter but the API service is
   configured for default_schedule_zone then by default
   the server will be scheduled to that zone.
3. The shelved offloaded server was unshelved by specifying the
   availability_zone with the
   POST /servers/{server_id}/action request using microversion
   2.77 or greater.
4. cinder.cross_az_attach is False, default_schedule_zone is None, the
   server is created without an explicit zone but with pre-existing volume
   block device mappings. In that case the server will be created in the
   same zone as the volume(s) if the volume zone is not the same as default_availability_zone. See Resource affinity for details.

If the server was not created in a specific zone then it is free to
be moved to other zones, i.e. the AvailabilityZoneFilter <AvailabilityZoneFilter>
is a no-op.

Knowing this, it is dangerous to force a server to another host with
evacuate or live migrate if the server is restricted to a zone and is
then forced to move to a host in another zone, because that will create
an inconsistency in the internal tracking of where that server should
live and may require manually updating the database for that server. For
example, if a user creates a server in zone A and then the admin force
live migrates the server to zone B, and then the user resizes the
server, the scheduler will try to move it back to zone A which may or
may not work, e.g. if the admin deleted or renamed zone A in the
interim.

Resource affinity

The cinder.cross_az_attach configuration
option can be used to restrict servers and the volumes attached to
servers to the same availability zone.

A typical use case for setting cross_az_attach=False is
to enforce compute and block storage affinity, for example in a High
Performance Compute cluster.

By default cross_az_attach is True meaning that the
volumes attached to a server can be in a different availability zone
than the server. If set to False, then when creating a server with
pre-existing volumes or attaching a volume to a server, the server and
volume zone must match otherwise the request will fail. In addition, if
the nova-compute service creates the volumes to attach to the server
during server create, it will request that those volumes are created in
the same availability zone as the server, which must exist in the block
storage (cinder) service.

As noted in the Implications for moving
servers section, forcefully moving a server to another zone could
also break affinity with attached volumes.

Using availability
zones to select hosts

We can combine availability zones with a specific host and/or node to
select where an instance is launched. For example:

$ openstack server create --availability-zone ZONE:HOST:NODE ... SERVER

However, as discussed previously, when
launching instances in this manner the scheduler filters are not run.
For this reason, this behavior is considered legacy behavior and,
starting with the 2.74 microversion, it is now possible to specify a
host or node explicitly. For example:

$ openstack --os-compute-api-version 2.74 server create \
    --host HOST --hypervisor-hostname HYPERVISOR ... SERVER

This avoids the need to explicitly select an availability zone and
ensures the scheduler filters are not bypassed.

Usage

Creating an availability zone (AZ) is done by associating metadata
with a host aggregate </admin/aggregates>. For this
reason, the openstack client provides the ability to create a
host aggregate and associate it with an AZ in one command. For example,
to create a new aggregate, associating it with an AZ in the process, and
add host to it using the openstack client, run:

$ openstack aggregate create --zone my-availability-zone my-aggregate
$ openstack aggregate add host my-aggregate my-host

Alternatively, you can set this metadata manually for an existing
host aggregate. For example:

$ openstack aggregate set \
    --property availability_zone=my-availability-zone my-aggregate

To list all host aggregates and show information about a specific
aggregate, in order to determine which AZ the host aggregate(s) belong
to, run:

$ openstack aggregate list --long
$ openstack aggregate show my-aggregate

Finally, to disassociate a host aggregate from an availability zone,
run:

$ openstack aggregate unset --property availability_zone my-aggregate

Configuration

Refer to /admin/aggregates for information on configuring both
host aggregates and availability zones.