Resolution: Not A Problem
Affects Version/s: 0.20.2
Fix Version/s: None
hadoop-0.20-0.20.2+228-1, centos 5, distcp
Datanodes register using their dns name (even configurable with dfs.datanode.dns.interface). However, the Namenode only really uses the source address that the registration came from when sharing it to clients wanting to write to HDFS.
Specific environment that causes this problem:
- Datanode and Namenode multihomed on two networks.
- Datanode registers to namenode using dns name on network #1
- Client (distcp) connects to namenode on network #2 (*) and is told to write to datanodes on network #1, which doesn't work for us.
(*) Allowing contact to the namenode on multiple networks was achieved with a socat proxy hack that tunnels network#2 to network#1 port 8020. This is unrelated to the issue at hand.
The cloudera link above recommends proxying for other reasons than multihoming, but it would work, but it doesn't sound like it would well (bandwidth, multiplicity, multitenant, etc).
Our specific scenario is wanting to distcp over a different network interface than the datanodes register themselves on, but it would be nice if both (all) interfaces worked. We are internally going to patch hadoop to roll back parts of the patch mentioned above so that we rely the datanode name rather than the socket address it uses to talk to the namenode. The alternate option is to push config changes to all nodes that force them to listen/register on one specific interface only. This helps us work around our specific problem, but doesn't really help with multihoming.
I would propose that datanodes register all interface addresses during the registration/heartbeat/whatever process does this and hdfs clients would be given all addresses for a specific node to perform operations against and they could select accordingly (or 'whichever worked first') just like round-robin dns does.