HBASE-12295, we have block with MemoryType.SHARED or MemoryType.EXCLUSIVE, the block in offheap BucketCache will be shared, and have an reference count to track its life cycle. If no rpc reference to the shared block, then the block can be evicted.
while after the
HBASE-21916, we introduced an refcount for ByteBuff, then I think we can unify the two into one. tried to fix this when preparing patch for HBASE-21879, but seems can be different sub-task, and it won't affect the main logic of HBASE-21879, so create a seperate one.