Clarifying what "Availability Zone" Means in Metacloud

In this solution:

The term "Availability Zone" has two definitions in Metacloud, and this can cause confusion. It is helpful to know the context for each definition so that you can:

  • Understand the purposes and capabilities of Availability Zones (AZs) in each context.
  • Troubleshoot and manage AZs in each context.
  • Communicate more clearly with Metacloud Support about AZs in each context.

Note: In Metacloud, "Availability Zone" is used differently from Amazon Web Services (AWS), where it refers to an isolated location within a geographical region.

"Availability Zone" as a Failure Domain

In the context of Metacloud physical infrastructure, an AZ is a failure domain, or a physical cluster of machines in your datacenter that host the processes running in your cloud. This includes Metacloud Control Planes (MCPs), Metacloud Hypervisors (MHVs), routers, and switches, as well as external storage devices if your organization uses them.

When Metacloud Support communicates with you about maintaining or troubleshooting AZs in this context, they may refer to them as "clusters" or "environments".

"Availability Zone" as a Cloud Concept

In the context of Metacloud operations, AZs are logical groupings of devices assembled for various purposes. Metacloud administrators can view these groupings in the Metacloud Dashboard or by using the command-line interface (CLI) or Metacloud API. 

Internal and Default AZs

Your initial view of your Metacloud environment includes two logical AZs created by the Metacloud Support Team during the build process. Only Support can alter these two AZs in any way:

  • The internal AZ, includes the Metacloud Control Planes (MCPs), which handle the scheduling and orchestration functions of your cloud.
  • The default AZ contains the Metacloud Hypervisors (MHVs), which run the instances created in your environment. The name of this AZ typically contains three to five letters selected by your organization.

The following screenshot shows these two AZs in a Metacloud Dashboard:


AZs Based on Host Aggregates

Metacloud administrators can add logical AZs, which are actually host aggregates, to your virtual environment. A host aggregate is a unique grouping of MHVs with associated metadata that an administrator can define for various purposes. One application might be to support different device grouping subsets.

For example, your organization could have one host aggregate for instances running Solid State Drive (SSD) storage and another for instances running Hard Disk Drive (HDD) storage. Or you can associate host aggregates with specific flavors or images, depending on their varying capacities or operating systems. Then, users can launch instances based on those flavors or images. 

Host aggregates are not visible in Metacloud to non-administrative users; however, administrators can "expose" a host aggregate by labeling it an "Availability Zone," using the Compute service (Nova). For more information, see Creating and Managing Host Aggregates.

The following screenshot shows two host aggregates that have been labeled as AZs and two that have not.

Note: An MHV can be associated with multiple host aggregates, but it can be associated with only one AZ that is based on a host aggregate. As the following screenshot shows, mhv3* and mhv5* are included in two host aggregates that are not tagged as AZs (host_agg_mg_3 and host_agg_mg_4). They are only included in one AZ that is based on a host aggregate, az_mg_2.


As mentioned earlier, your newly delivered cloud includes a default AZ, which contains all the MHVs to be used for hosting instances. 

Attaching Instances to Logical AZs

When launching instances, non-administrative users can associate them with the default AZ or any AZs associated with host aggregates. If a user associates an instance with a specific AZ when launching it, that AZ is permanently associated with the record of the instance in the Metacloud database. 

If a user selects the option for Any Availability Zone when using the Dashboard to launch an instance, or does not specify an AZ when using the command-line interface (CLI), the Compute service "assigns" the instance to the AZ with the most available hosting resources; however, the instance shows a value of NULL for Availability Zone in the database and is not permanently associated with any AZ. 

The following screenshot shows the launching of an instance in the Dashboard with Any Availability Zone selected.


See Launching Images Using the Dashboard or Selecting Hosts to Launch Instances for more information about launching instances.

For more information about how host aggregates and AZs work with instances, see Troubleshooting VMs From Deleted Host Aggregates Tagged as AZs.

If you are a Metacloud administrator, and you are reporting issues to Support regarding AZs created with the Compute service, it is helpful to refer to them as "host aggregates" to distinguish them from the default AZ.  



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