Host aggregates and availability zones in Openstack

 HOST AGGREGATES AND AVAILABILITY ZONES:

Host Aggregates:

It is a mechanism by which we separate hosts and form a group of hosts in Openstack cloud. this grouping decision can be made on the basis of additional arbitrary characteristics. Host aggregates are only known to administrators. So, Administrators map flavors to host aggregates by setting some metadata on the host aggregates. The scheduler is used to match user requests for the instance of given flavor of a host aggregate with the same key value pair in its metadata. 

 Availability Zones:

Unlike host aggregates, they can be explicitly exposed to user. Only administrator have this right as a option in which host aggregates can be exposed as availability zone. Administrators can configure a default availability zone where instances will be scheduled when the user fails to specify one.  

 COMMAND-LINE INTERFACE
The Nova command-line tool supports the following aggregate-related commands.

• nova aggregate-list 

     This command will list all the aggregates

•  nova aggregate-create <name> [availability-zone]

   Create a new aggregate named <name>, and optionally in availability zone [availability-zone] if specified. The command returns the ID of the newly created aggregate. Hosts can be made available to multiple host aggregates. Be careful when adding a host to an additional host aggregate when the host is also in an availability zone. Pay attention when using the aggregate-set-metadata and aggregate-update commands to avoid user confusion when they boot instances in different availability zones. An error occurs if you cannot add a particular host to an aggregate zone for which it is not intended.

• nova aggregate-delete <id>

         This will delete an aggregate with id <id>

• nova aggregate-details <id>

         Show details of the aggregate with id <id>

• nova aggregate-add-host <id> <host>

         Add host with name <host> to aggregate with id <id>  

•  nova aggregate-remove-host <id> <host>

Remove the host with name <host> from the aggregate with id <id>.

• nova aggregate-set-metadata <id> <key=value> [<key=value> ...]

Add or update metadata (key-value pairs) associated with the aggregate with id <id>.

• nova aggregate-update <id> <name> [<availability_zone>]

Update the name and availability zone (optional) for the aggregate.

• nova host-list

List all hosts by service.

• nova host-update --maintenance [enable | disable]

Put/resume host into/from maintenance.

  
 Configure scheduler to support host aggregates:

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