When you want to stop and restart OpenStack instances, there are four different options to consider.
First, admins can pause and unpause a Nova compute instance. When an instance is paused, the entire state of the instance is kept in RAM. Pausing an instance will disable access to that instance, but won't free up any of its resources.
Another option is to suspend, and then resume, an instance. Like paused OpenStack instances, a suspended instance keeps its current state, but it is written to storage. Suspension, which frees up an instance's resources, is a better choice for instances that an organization doesn't need for a longer period of time.
A third option is to shelve OpenStack instances. Where a suspended instance maintains all the information it currently has in RAM, this is not the case for a shelved instance. A shelved instance, however, does keep all of its associated resources. So if the instance uses ephemeral storage, it maintains that resource when an admins shelves it, and continues to use that resource when unshelved.
A shelved instance is actually shut down, which is not the case for suspended or paused instances. If admins decide they no longer need a shelved instance, they can remove it, which ensures that it doesn't maintain any hypervisor-level resources in use.
The last option is to stop an instance in Nova, which will disconnect all of its associated resources. This means admins can't restore a stopped instance to its previous state. This option is only useful for OpenStack instances that an organization no longer needs. In all other cases, admins should shelve, suspend or pause the instance.
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