If there is a datanode/network failure in the write pipeline, DFSClient will try to
remove the failed datanode from the pipeline and then continue writing with the
remaining datanodes. As a result, the number of datanodes in the pipeline is decreased.
The feature is to add new datanodes to the pipeline. This is a site-wide property to
enable/disable the feature. When the cluster size is extremely small, e.g. 3 nodes or less,
cluster administrators may want to set the policy to NEVER in the default configuration
file or disable this feature. Otherwise, users may experience an unusually high rate of
pipeline failures since it is impossible to find new datanodes for replacement. See also