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From Virajith Jalaparti <virajit...@gmail.com>
Subject Lack of data locality in Hadoop-0.20.2
Date Tue, 12 Jul 2011 12:45:26 GMT

I was trying to run the Sort example in Hadoop-0.20.2 over 200GB of input
data using a 20 node cluster of nodes. HDFS is configured to use 128MB block
size (so 1600maps are created) and a replication factor of 1 is being used.
All the 20 nodes are also hdfs datanodes. I was using a bandwidth value of
50Mbps between each of the nodes (this was configured using linux "tc"). I
see that around 90% of the map tasks are reading data over the network i.e.
most of the map tasks are not being scheduled at the nodes where the data to
be processed by them is located.
My understanding was that Hadoop tries to schedule as many data-local maps
as possible. But in this situation, this does not seem to happen. Any reason
why this is happening? and is there a way to actually configure hadoop to
ensure the maximum possible node locality?
Any help regarding this is very much appreciated.


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