Ceph » bluestore

My cluster configuration is 10 BlueStore OSDs, ec overwrite 8+2, BlueStore_cache_size（only onode cache） = 16MB (for reduce memory consumption).I created 30 RBD image and wrote these by librbd of python.After a while,I checked dump_mempools,found the sum of bluestore_cache_onode and bluestore_cache_other are more than 200MB.So I opened debug_bluestore to 20,the log is like as:
2017-09-21 21:12:10.588694 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c8af972f:::*inc_osdmap*.80:0# has 2 refs; skipping
2017-09-21 21:12:10.588700 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:374ce826:::*osdmap*.80:0# has 2 refs; skipping
2017-09-21 21:12:10.588703 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c551972f:::inc_osdmap.81:0# has 2 refs; skipping
2017-09-21 21:12:10.588706 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:3dcce826:::osdmap.81:0# has 2 refs; skipping
2017-09-21 21:12:10.588708 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:ce51972f:::inc_osdmap.82:0# has 2 refs; skipping
2017-09-21 21:12:10.588710 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:32cce826:::osdmap.82:0# has 2 refs; skipping
2017-09-21 21:12:10.588714 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c0d1972f:::inc_osdmap.83:0# has 2 refs; skipping
2017-09-21 21:12:10.588716 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:380ce826:::osdmap.83:0# has 2 refs; skipping
2017-09-21 21:12:10.588718 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:cb11972f:::inc_osdmap.84:0# has 2 refs; skipping
2017-09-21 21:12:10.588721 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:358ce826:::osdmap.84:0# has 2 refs; skipping
2017-09-21 21:12:10.588724 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c611972f:::inc_osdmap.85:0# has 2 refs; skipping
2017-09-21 21:12:10.588726 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:3e8ce826:::osdmap.85:0# has 2 refs; skipping
2017-09-21 21:12:10.588728 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:cc91972f:::inc_osdmap.86:0# has 2 refs; skipping
2017-09-21 21:12:10.588730 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:306ce826:::osdmap.86:0# has 2 refs; skipping
2017-09-21 21:12:10.588732 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c371972f:::inc_osdmap.87:0# has 2 refs; skipping
2017-09-21 21:12:10.588735 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:3bece826:::osdmap.87:0# has 2 refs; skipping
2017-09-21 21:12:10.588738 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c9f1972f:::inc_osdmap.88:0# has 2 refs; skipping
2017-09-21 21:12:10.588740 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:36ece826:::osdmap.88:0# has 2 refs; skipping
2017-09-21 21:12:10.588742 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c4f1972f:::inc_osdmap.89:0# has 2 refs; skipping
2017-09-21 21:12:10.588744 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:3c2ce826:::osdmap.89:0# has 2 refs; skipping
2017-09-21 21:12:10.588747 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:c1b1972f:::inc_osdmap.90:0# has 2 refs; skipping
2017-09-21 21:12:10.588750 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:39536826:::osdmap.90:0# has 2 refs; skipping
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2017-09-21 21:12:10.588758 7fe3a5bf2700 20 bluestore.2QCache(0x7fe3c185c2a0) _trim #-1:39536826:::superblock# has 2 refs; skipping

Makebe all Onode of coll_t::meta will not be unref? but why?