I use C* v2.1.2 do write only workload (into one column family) and experiment with offheap-objects memtable space.
I set parameters to:
memtable_offheap_space_in_mb = 51200 # 50Gb memtable_cleanup_threshold = 0.99
and expect that flush will not be triggered until available memtable offheap space reaches ~50Gb. But flushes are triggered before that limit. System monitor shows that in used only ~16Gb at that moment (linux+jvm+heap+...).
Why such thing is happened?