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From Apache Wiki <wikidi...@apache.org>
Subject [Hadoop Wiki] Update of "TroubleShooting" by WynandWinterbach
Date Sat, 11 Jul 2009 12:06:43 GMT
Dear Wiki user,

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The following page has been changed by WynandWinterbach:

The comment on the change is:
Added tips for hanging reduce steps (I ran into these issues).

   * There may not be enough (any) datanodes for the data to be written. Again, check the
   * The datanodes on which the blocks were stored might be down. 
+ == Reduce hangs ==
+ This can be a DNS issue. Two problems which have been encountered in practice are:
+  * Machines with multiple NICs. In this case, set dfs.datanode.dns.interface (in hdfs-site.xml)
and mapred.datanode.dns.interface (in mapred-site.xml) to the name of the network interface
used by Hadoop (something like eth0 under Linux),
+  * Badly formatted hosts files (/etc/hosts under Linux) can wreak havoc. Any DNS problem
will hobble Hadoop, so ensure that names can be resolved correctly.

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