swift charms need support for removing devices

[Jacek Nykis]

The swift-proxy charm generates ring files using data from the swift-storage relation which provides list of disks on each storage node.
The swift-storage charm will only relation-set list of disks in swift-storage-relation-joined hook.

This is problematic because it means it's impossible to permanently remove disk from swift (it will be readded next time the swift-storage relation runs) .

It may be possible to change weights or add disks by hand on the storage-proxy unit but I am not entirely sure this changes will be persistent, they should be but I have not read the whole charm to be absolutely sure.

[Gareth Woolridge (moon127)]

I hit similar issue to this yesterday when removing a node with a single disk from a zone with juju remove-unit the rings were not updated to reflect the removed node/disk.

While the rings were in this inconsistent state it was possible to add-unit and the additional node/disk was added to the ring files and joined the cluster successfully.

However once I manually removed the previously removed node/disk from the rings subsequent add-unit operations caused a hook error on the swift-proxy instance and a traceback in that unit's juju logs.

So it appears that manually mixing manual maintenance operations with juju managed service causes issues, but also currently we support scale-out but not scale-in.

[Edward Hope-Morley (hopem)]

Since support for juju storage is being added for the 16.04 release of the charms and there are some related fixes under way (e.g. bug 1383390) is probably a good time to bring this one to fore once again.