Janus Chow created HDFS-15738:
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Summary: Forbid the transition to Observer state when NameNode is
in StartupSafeMode
Key: HDFS-15738
URL: https://issues.apache.org/jira/browse/HDFS-15738
Project: Hadoop HDFS
Issue Type: Improvement
Reporter: Janus Chow
Currently when a _getBlockLocation_ request comes to an Observer Namenode which
is in safemode, NameNode will have a check that if the result is empty, it will
reply to the client with a _RetriableException_, noting the client to retry the
request later.
And If the Observer Namenode is in startup safe mode, the client would have to
wait for the Observer NameNode to leave the safe mode. For a big cluster, it
may cause a long time of waiting for the client. In our cluster, we met this
problem, and the client needs to wait for about 30 minutes before the service
back to normal.
The reason for this situation is that the NameNode becomes the state of
Observer when it's still in safe mode getting Datanode's block reports. And
here are two solutions for this issue:
# Throw _ObserverRetryOnActiveException_ when the Observer NameNode is in
startup safe mode, redirecting the user's requests to active NN.
# Forbid the transition to Observer state when the cluster maintainer is
trying to do the transition operation.
We choose the second solution because the first one would abet the bad
operation of transition NN to Observers while it's not ready for real service.
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