Conceptually you can think of the namenode is similar to a journal file
system. For each write, it updates the in-memory data structure, persists
the operations on the stable storage (i.e., calling sync to flush the
buffer of the edit logs), then responds to the client.

Note that all writes are serialized, which means the writes are given a
total order. There are no consistent issues between multiple clients.

For question 3, the NN can writes to multiple edit logs  with the same
content at the same time. This allows the operator to store a copy of edit
logs in NFS. In this case NN calls sync() for each edit log.

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