Storage location balancing

[jjg]

Extend storage location config to include additional attributes:

• read-only: don't write additional blocks to this location
• drain: move blocks from this location to others
• read-write: read and write blocks in this location (default)

Notes

drain should include an extended attribute which is the drain rate expressed in blocks-per-minute. This target rate is only a "best effort" and can vary depending on many factors, but will not be exceeded.

If drain is based on reading inodes first, it should also provide garbage collection as blocks that are no longer referenced by inodes will be left behind on the drained storage location.