Andre Araujo created HDFS-12643:
-----------------------------------
Summary: HDFS maintenance state behaviour is confusing and not
well documented
Key: HDFS-12643
URL: https://issues.apache.org/jira/browse/HDFS-12643
Project: Hadoop HDFS
Issue Type: Bug
Components: documentation, namenode
Reporter: Andre Araujo
The current implementation of the HDFS maintenance state feature is confusing
and error-prone. The documentation is missing important information that's
required for the correct use of the feature.
For example, if the Hadoop admin wants to put a single node in maintenance
state, he/she can add a single entry to the maintenance file with the contents:
{code}
{
"hostName": "host-1.example.com",
"adminState": "IN_MAINTENANCE",
"maintenanceExpireTimeInMS": 1507663698000
}
{code}
Let's say now that the actual maintenance finished well before the set
expiration time and the Hadoop admin wants to bring the node back to NORMAL
state. It would be natural to simply change the state of the node, as show
below, and run another refresh:
{code}
{
"hostName": "host-1.example.com",
"adminState": "NORMAL"
}
{code}
The configuration file above, though, not only take the node {{host-1}} out of
maintenance state but it also *blacklists all the other DataNodes*. This
behaviour seems inconsistent to me and is due to {{emptyInServiceNodeLists}}
being set to {{false}}
[here|https://github.com/apache/hadoop/blob/230b85d5865b7e08fb7aaeab45295b5b966011ef/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/CombinedHostFileManager.java#L80]
only when there is at least one node with {{adminState = NORMAL}} listed in
the file.
I believe that it would be more consistent, and less error prone, to simply
implement the following:
* If the dfs.hosts file is empty, all nodes are allowed and in normal state
* If the file is not empty, any host *not* listed in the file is *blacklisted*,
regardless of the state of the hosts listed in the file.
Regardless of the implementation being changed or not, the documentation also
needs to be updated to ensure the readers know of the caveats mentioned above.
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