Requesting MongoDB to officially support "LRU Capped Collections", basically capped collections but with LRU instead of FIFO ordering. IOW I would like to be able to set a max collection size, and have MongoDB automatically delete least recently used (read or write) documents when full. The LRU need not be perfect, but a decent approximation would likely be good enough for most needs.

This is a fundamental requirement for us using MongoDB as a cache, presently we will need to implement one of the following less desireable hacks:
1) Maintain the LRU in app server memory (sucks in a clustered environment)
2) Maintain a "last access" timestamp with every doc (sucks in that it turns every read into a write)

Either way, we have to write a background job that continually monitors the db size and deletes when we hit a threshold (or do this on every write which is expensive). Which also sucks because it has ugly failure modes.

Ideally, the LRU would be persisted. I can think of a few ways this can be done relatively cheaply, as long as the LRU is not expected to be exact. A simple in-memory lru would be sufficient for our needs though.

A couple of things I don't need, though others might
1) Support for sharded collections. I feel that a per-shard LRU is plenty good, no need to maintain perfect LRU behaviour across all servers.
2) LRU across collections, which would add complex inter-collection behaviour

Thanks and open to alternatives. I tried seeing how this could be simulated with capped collections but could not find a reasonable solution.