Dear Wiki user, You have subscribed to a wiki page or wiki category on "Hadoop Wiki" for change notification.
The following page has been changed by WynandWinterbach: http://wiki.apache.org/hadoop/TroubleShooting 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 logs. * 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. +
