Quick tip on how to retrieve cache statistics from the a cache pool.
Simply use the admin socket:
$ sudo ceph daemon osd.{id} perf dump
...
"tier_promote": 1425,
"tier_flush": 0,
"tier_flush_fail": 0,
"tier_try_flush": 216,
"tier_try_flush_fail": 21,
"tier_evict": 1413,
"tier_whiteout": 201,
"tier_dirty": 671,
"tier_clean": 216,
"tier_delay": 16,
...