private boolean canMerge(List<Long> olderDataFiles){
		int spaceRate=0;
		int nullCount=0;
		if(olderDataFiles == null)
			return false;
		if(olderDataFiles.size() < HdbConst.MERGE_FILE_SIZE)
			return false;
		
		if(olderDataFiles.size() > HdbConst.FORCE_MERGE_FILE_SIZE)
			return true;
		
		for(int index=0;index<olderDataFiles.size();index++){
			DataFileCounter dataFileCounter=DataFileCounterManager.get(index);
			if(dataFileCounter != null)
				spaceRate+=dataFileCounter.SpaceRate();
			else
				nullCount++;
		}
		
		if(spaceRate/(olderDataFiles.size() - nullCount) > 0.5)
			return false;
		else
			return true;
	}