Ceph

The reason we haven't done this yet is because it's a hard UI problem. We need to define how to present the information to the user, and what kinds of lies we want to tell. If they have one pool with 2x replication and one with 3x replication, how are we supposed to use those different multipliers to present a single available space number? (This will get a lot harder once we have erasure coding pools, too!)
It's easy to say "oh, all pools have the same multiplier", but we can't really provide that output for only one one specific subset of user configurations.

I am thinking in the context of alerts, knowing when any pool in the cluster in running out of usable space has value so just being able to show stats on a per-pool basis initially would be useful.

I take your point about the aggregate cluster issues.. blending information from multiple pools doesn't really help with capacity management (which might be the main use-case for showing aggregate usable space).