Ceph

Currently cache tier's flushing/evicting triggering mechanism is based on dirty ratio and full dirty ratio. This is useful to control cache tier pool's space utilization.

However one user case that we encountered is that customer doesn't care too much on such ratio towards their requirement. Customers want to move cold data to cheap base pool from upper expensive but small cache tier pool. They want to see only hot data staying at cache tier pool.

So we add such feature on cache tier with a configurable option, together with cache_min_flush_age, osd agent can flush objects which meet cache_min_flush_age to base pool periodically and then evict them.