How namenode will map to physical ram


#If namenode and datanode are external devices(vitural) ,what is the role of physical ram.
#As per the course discussion the namenode will contain metadata,so,fetching any read/write from namenode(virtual) is a perfomance bottleneck .
#for example if I want to fetch any data how exactly the mapping will be happen from namenode(virtual) to physical ram.

Correct me if I am missing any thing