Spawned from RFD 312

• Tracking the right info in the DB
  • Store the "maximum allowed size" of a dataset
  • Add constraints where appropriate to limit this size (e.g., datasets < zpool size, regions < dataset size)
• Quotas & Reservations
  • (Nexus) Determine what Quotas & Reservations should be for all known datasets
  • (Nexus) Request that quotas/reservations are set in a request to the Sled Agent
  • (Sled Agent) Actually apply Quotas & Reservations
  • (Nexus) Request a "ballast" for reserved capacity explicitly
• Scaling
  • (Sled Agent) Add metric to periodically monitor space usage
  • (Nexus) Periodically query space usage of datasets, and take actions at certain thresholds...
    • ... When expected, perform horizontal scaling
    • ... Identify low-space conditions (long-term, we should be providing an alert to operators if we cannot remedy the situation ourselves).