CephFS does that. You can use getfattr to ask it for the recursive number of entries or bytes in a given directory. Querying it is constant time, updates upda

CephFS does that. You can use getfattr to ask it for the recursive number of entries or bytes in a given directory. Querying it is constant time, updates update it with a few seconds delay. Extremely useful when you have billions of files on spinning disks, where running du/ncdu would take a month just for the stat()s.