Why does the Hadoop incompatible namespaceIDs issue happen?

Question

This is a fairly well-documented error and the fix is easy, but does anyone know why Hadoop datanode NamespaceIDs can get screwed up so easily or how Hadoop assigns the NamespaceIDs when it starts up the datanodes?

Here's the error:

2010-08-06 12:12:06,900 ERROR org.apache.hadoop.hdfs.server.datanode.DataNode: java.io.IOException: Incompatible namespaceIDs in /Users/jchen/Data/Hadoop/dfs/data: namenode namespaceID = 773619367; datanode namespaceID = 2049079249
    at org.apache.hadoop.hdfs.server.datanode.DataStorage.doTransition(DataStorage.java:233)
    at org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:148)
    at org.apache.hadoop.hdfs.server.datanode.DataNode.startDataNode(DataNode.java:298)
    at org.apache.hadoop.hdfs.server.datanode.DataNode.<init>(DataNode.java:216)
    at org.apache.hadoop.hdfs.server.datanode.DataNode.makeInstance(DataNode.java:1283)
    at org.apache.hadoop.hdfs.server.datanode.DataNode.instantiateDataNode(DataNode.java:1238)
    at org.apache.hadoop.hdfs.server.datanode.DataNode.createDataNode(DataNode.java:1246)
    at org.apache.hadoop.hdfs.server.datanode.DataNode.main(DataNode.java:1368)

This seems to even happen for single node instances.

Answer 1

I was getting this too, and then I tried putting my configuration in hdfs-site.xml instead of core-site.xml.

Seems to stop and start without that error now.

[EDIT, 2010-08-13]

Actually this is still happening, and it is caused by formatting.

If you watch the VERSION files when you do a format, you'll see (at least I do) that the namenode gets assigned a new namespaceID, but the data node does not.

Quick solution is to delete the VERSION for the datanode before format.

[TIDE, 2010-08-13]

Answer 2

Namenode generates new namespaceID every time you format HDFS. I think this is possibly to differentiate current version and previous version. You can always rollback to previous version if something is not proper which may not be possible if namespaceID is not unique for every formatted instance.

NamespaceID also connects namenode and datanodes. Datanodes bind themselves to namenode through namespaceID