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From "Agarwal, Nikhil" <Nikhil.Agar...@netapp.com>
Subject Map Tasks do not obey data locality principle........
Date Tue, 14 May 2013 10:55:26 GMT
Hi,

I  have a 3-node cluster, with JobTracker running on one machine and TaskTrackers on other
two (say, slave1 and slave2). Instead of using HDFS, I have written my own FileSystem implementation.
Since, unlike HDFS I am unable to provide a shared filesystem view to JobTrackers and TaskTracker
thus, I mounted the root container of slave2 on a directory in slave1 (nfs mount). By doing
this I am able to submit MR job to JobTracker, with input path as my_scheme://slave1_IP:Port/dir1,
etc.  MR runs successfully but what happens is that data locality is not ensured i.e. if files
A,B,C are kept on slave1 and D,E,F on slave2 then according to data locality, map tasks should
be submitted such that map task of A,B,C are submitted to TaskTracker running on slave1 and
D,E,F on slave2. Instead of this, it randomly schedules the map task to any of the tasktrackers.
If map task of file A is submitted to TaskTracker running on slave2 then it implies that file
A is being fetched over the network by slave2.

How do I avoid this from happening?

Thanks,
Nikhil



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