An 'intermittent' target that has priority reconciling #1030
Description
This quote from the 1.33.0 release notes made me think of something:
This also means you can create a single device filesystem with replicas=2, and upon adding a new device data will automatically be replicated on the new device, no additional user intervention required.
Would it be possible to add some drives or groups of drives as 'intermittent' such that they can be disconnected/powered off for most of the time without affecting replica/degradation count (as if they had durability=0) but when they are connected they have everything immediately synchronized over (as if the rest of the system had durability=0). Disconnecting these 'intermittent' drives does not degrade any data, they're a non-vital mirror.
A simple use case would be backup: I have a laptop and plug in a drive that I want to back up onto, this happens automatically. On a bigger scale I could power down a lot of drives 23/24h a day or more.
An extension would be to allow files/directories to be marked as archived, they are on the 'intermittent' drives but not on the live ones. Metadata is kept but immutable, they are in the file tree but the data is inaccessible. Maybe you could allow some operations that are only performed when the 'intermittent' drives are re-connected like queuing up deletes.
It could also be possible for the files to still be kept on the live ones until the space is needed. They are cached but overwritten if space is needed.
The manual solution would be to use the future send/receive features between two pools. For a full system that would be easy, but the above would work with existing granularity: I can have specific directories and files transfer without needing a file with a dozen target-destination pairs to remember.