Ceph

build a model that includes

- dirty bytes value
 - dirty files values
 - a cost function of bytes and inodes
 - an lru
 - a writeback initiator with some threshold (analogous to dirty_background_bytes)
 - a writeback blocker with some threshold (analogous to dirty_bytes)

replace the current background flush thing with the above.

set sane defaults for btrfs, xfs that roughly model how expensive inode vs content writes are