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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