Support for cascade-style destinations

[trhr]

For those of us with hotswappable drives, it'd be nice to actually fill drives up rather than writing equally to the entire array. I see two potential configurations:

• Perfect cascade. Write only to dest1 until full. Then, write to dest2 until full. Then, write until dest3 until full, and so on.
• Optimized cascade. Write to dest1 only if dest1's i/o is below some specified value (e.g. a plot is being written here). If dest1 is in use, write to dest2, then dest3, then dest4, and so on.