When using a cloud, it is understandable that a “machine” would be removed (destroyed) when there is no longer an app/unit running on it. But for the manual case, it is not the same.
Currently, for the manual case, when a machine becomes empty, it is automatically removed from the list of available machines and is left in a state that prevents it from being reused with a simple “add-machine” as previously (an add-machine returns “ERROR machine is already provisioned”). A simple “rm” of specific files cleans this up. Nevertheless, being able to specify that said machine be persisted would be helpful.
A workaround is to put a “keepalive” app/unit on the machine, but this seems a bit hacky.
I can envision a situation in which a persistent option would also be useful in the cloud, too.