Which two updates occur when a client application opens a stream to begin a file write on a cluster
running MapReduce v1 (MRv1)?

A.
Once the write stream closes on the DataNode, the DataNode immediately initiates a black
report to the NameNode.

B.
The change is written to the NameNode disk.

C.
The metadata in the RAM on the NameNode is flushed to disk.

D.
The metadata in RAM on the NameNode is flushed disk.

E.
The metadata in RAM on the NameNode is updated.

F.
The change is written to the edits file.

Explanation:
Note: Namenode stores modifications to the filesystem as a log appended to a
native filesystem file (edits). When a Namenode starts up, it reads HDFS state from an image file
(fsimage) and then applies edits from edits log file. It then writes new HDFS state to (fsimage) and
starts normal operation with an empty edits file. Since namenode merges fsimage and edits files
only during start up, edits file could get very large over time on a large cluster. Another side effect
of larger edits file is that next restart of Namenade takes longer.
The secondary namenode merges fsimage and edits log periodically and keeps edits log size with
in a limit. It is usually run on a different machine than the primary Namenode since its memory
requirements are on the same order as the primary namemode. The secondary namenode is
started by bin/start-dfs.sh on the nodes specified in conf/masters file.