namenode how to keep the latest file directory tree structure

namenode will modify the records stored in editlog , the directory tree stored in fsimage in , secondnamenode editlog every update to the fsimage the 3600s , the problem came ?

why we can see through to the latest namenode directory tree structure? ( rather secondnamenode update at least one hour delay )?
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namenode only loaded at boot time fsimage, and replay editlog, after all modifications are accessing the metadata in memory to modify , and to manipulate recorded editlog in
secondarynamenode one for cold standby , the second is to reduce namenode restart replay editlog next time ,
This is no direct relationship between the two