Availability Zones (AZs) are a special case of host aggregates. Here, host aggregates are used by Nova to make scheduling decisions; they are generally only visible to operators and administrators of the OpenStack Cloud. AZs are the end-user visible component. An AZ can be configured, like host aggregates, to represent features of the hardware.
AZs are typically used to define failure domains, such as a cabinet or data center, or even geography. When configuring AZs, it is important to note that a host can only be a member of one AZ at a time.
Creating an AZ is a two-step process. First, we create an aggregate with the --zone parameter, or add it to an existing aggregate. Second, a host needs to be added to the aggregate before instances can be scheduled into the zone.
To create an AZ, you will need the following information:
- The
openstackcommand-line client - The
openrcfile containing appropriate credentials - The name or ID of the aggregate
- The desired name of the AZ
- The host or hosts to add to the AZ
For our example, these values are as follows:
- Aggregate name:
cookbook-az - Availability zone name:
cookbook-az - Compute host:
compute-01
The following steps are to be used when creating an AZ:
- First, we will list the current AZs with the following command:
openstack availability zone listThis will bring back an output like the following:
- Create a new aggregate with the
--zoneparameter:openstack aggregate create --zone cookbook-az cookbook-azThis will bring back an output like the following:
- Now add a host to the aggregate with the following command:
openstack aggregate add host cookbook-az compute-01This will bring back an output like the following when successful:
- Now list the AZs we have available:
openstack availability zone listThis will show the following:
The first command, openstack availability zone list, lists all AZs. Next, when creating the new aggregate, the special --zone cookbook-az parameter is passed, telling OpenStack Nova that this aggregate is also an AZ, specifying its name. Finally, a host is added to the AZ much the same as a host is added to an aggregate.