Description of problem:
Cache pool always makes sure that number of objects is less than target_max_object but not exactly the same. suppose if I set target_max_object as 50 no matter how many objects are pushed to the fast pool , around 48 objects will remain in the pool. 

Version-Release number of selected component (if applicable):

ceph-0.94.5-4.el7cp.x86_64
How reproducible:

always
Steps to Reproduce:
1.create a slow pool and a fast pool 
2.make fast pool as cache tier for slow pool
here is the config 

[ubuntu@magna105 ~]$ sudo ceph osd pool get fast hit_set_period
hit_set_period: 120  
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast hit_set_count
hit_set_count: 0
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast hit_set_fpp
hit_set_fpp: 0.05
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast target_max_objects
target_max_objects: 50
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast target_max_bytes
target_max_bytes: 0
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast cache_target_dirty_ratio
cache_target_dirty_ratio: 0.4
[ubuntu@magna105 ~]$ sudo ceph osd pool set fast cache_target_dirty_ratio 1
set pool 415 cache_target_dirty_ratio to 1
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast cache_target_full_ratio
cache_target_full_ratio: 1
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast cache_min_flush_age
cache_min_flush_age: 0
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast cache_min_flush_age
cache_min_flush_age: 0
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast min_read_recency_for_promote
min_read_recency_for_promote: 0
[ubuntu@magna105 ~]$ sudo ceph osd pool get fast write_fadvise_dontneed
write_fadvise_dontneed: false

3. create 100 objects by writing it to the slow pool and observer that approximately target_max_objects will stay in fast pool.

4. Now read 100 objects from slow pool and observe the number of objects in fast pool 

[ubuntu@magna105 ~]$ sudo rados -p fast ls   | wc -l
47


only 47 objects are preserved in fast pool . why the number of objects in the fast pool is always less than target_max_objects


Actual results:
cache tier will never have exact target_max_objects of objects.

Expected results:
fast pool should have exact target_max_objects of objects.

Additional info: