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From "Matthew Byng-Maddick (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-1379) Multihoming brokenness in HDFS
Date Tue, 07 Sep 2010 14:56:33 GMT
Multihoming brokenness in HDFS
------------------------------

                 Key: HDFS-1379
                 URL: https://issues.apache.org/jira/browse/HDFS-1379
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: hdfs client, name-node
    Affects Versions: 0.20.1
         Environment: Multi-homed namenode and datanodes. hadoop-0.20.1 (cloudera distribution
on linux)
            Reporter: Matthew Byng-Maddick


We have a setup where - because we only have a very few machines (4 x 16 core) we're looking
at co-locating namenodes and datanodes. We also have front-end and back-end networks. Set-up
is something like:

* machine1
** front-end 10.18.80.80
** back-end 192.168.24.40
* machine2
** front-end 10.18.80.82
** back-end 192.168.24.41
* machine3
** front-end 10.18.80.84
** back-end 192.168.24.42
* machine4
** front-end 10.18.80.86
** back-end 192.168.24.43

On each, the property *slave.host.name* is configured with the 192. address, (the *.dns.interface
settings don't actually seem to help, but that's a separate problem), and the *dfs.datanode.address*
is bound to the 192.168.24.x address on :50010, similarly the *dfs.datanode.ipc.address* is
bound there.

In order to get efficient use of our machines, we bring up a namenode on one of them (this
then rsyncs the latest namenode fsimage etc) by bringing up a VIP on each side (we use the
10.18.80.x side for monitoring, rather than actual hadoop comms), and binding the namenode
to that - on the inside this is 192.168.24.19.

The namenode now knows about 4 datanodes - 192.168.24.40/1/2/3. These datanodes know how they're
bound. However, when the datanode is telling an external hdfs client where to store the blocks,
it gives out 192.168.24.19:50010 as one of the addresses (despite the datanode not being bound
there) - because that's where the datanode->namenode RPC comes from.

This is wrong because if you've bound the datanode explicitly (using *dfs.datanode.address*)
then that's should be the only address the namenode can give out (it's reasonable, given your
comms model not to support NAT between clients and data slaves). If you bind it to * then
your normal rules for slave.host.name, dfs.datanode.dns.interface etc should take precedence.

This may already be fixed in later releases than 0.20.1 - but if it isn't it should probably
be - you explicitly allow binding of the datanode addresses - it's unreasonable to expect
that comms to the datanode will always come from those addresses - especially in multi-homed
environments (and separating traffic out by network - especially when dealing with large volumes
of data) is useful.

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