-RE: balancing and replication in HDFS
Jeffrey Buell 2011-02-26, 00:45
Thanks for the quick response.
I added <final>true</final> to dfs.replication, but I still get just one output copy. Can hadoop apps overwrite the replication level even with this parameter?
I tried increasing mapred.tasktracker.reduce.tasks.maximum from 1 to 4, but that didn't make any difference: output is still all on one node. I thought that parameter controls the number of reduce tasks per node so 1 should be sufficient, is that correct logic? The other reduce parameters are at their defaults. Which parameters should I be trying?
There's no way the sort can run efficiently if the gen is unbalanced. At 5 GB, there're plenty of blocks to make the distribution even. How does HDFS decide to spread the storage across the nodes? Note the sizes of the 4 chunks (1.7,...,3.2 GB) seem to be repeatable, but they land on different nodes each time teragen is run.
> -----Original Message-----
> From: Todd Lipcon [mailto:[EMAIL PROTECTED]]
> Sent: Friday, February 25, 2011 3:13 PM
> To: [EMAIL PROTECTED]
> Subject: Re: balancing and replication in HDFS
> Hi Jeff,
> The output of terasort has replication level 1 by default. This is so
> it goes faster with the default settings and makes for more impressive
> benchmark results :)
> The reason you see it all on one machine is probably that you're
> running with one reducer. Try configuring your terasort to use more
> reduce tasks and you should see the load (and space usage) even out.
> On Fri, Feb 25, 2011 at 2:52 PM, Jeffrey Buell <[EMAIL PROTECTED]>
> > I'm a newbie to hadoop and HDFS. I'm seeing odd behavior in HDFS
> that I hope somebody can clear up for me. I'm running hadoop version
> 0.20.1+169.127 from the cloudera distro on 4 identical nodes, each with
> 4 cpus and 100GB disk space. Replication is set to 2.
> > I run:
> > hadoop jar /usr/lib/hadoop/hadoop-*-examples.jar teragen 50000000
> > This produces the expected 10GB of data on disk (5GB * 2 copies).
> But the data is spread very unevenly across the nodes, ranging from
> 1.7 to 3.2 GB on each node. Then I sort the data:
> > hadoop jar /usr/lib/hadoop/hadoop-*-examples.jar terasort tera_in5
> > It finishes successfully, and HDFS recognizes the right amount of
> > $ hadoop fs -du /user/hadoop/
> > Found 2 items
> > 5000023410 hdfs://namd-1/user/hadoop/tera_in5
> > 5000170993 hdfs://namd-1/user/hadoop/tera_out5
> > However all the new data is on one node (apparently randomly chosen),
> and the total disk usage is only 15GB, which means that the output data
> is not replicated. For nearly all the elapsed time of the sort, the
> other 3 nodes are idle. Some of the output data is in
> dfs/data/current, but a lot is in one of 64 new subdirs
> (dfs/data/current/subdir0 through subdir63).
> > Why is all this happening? Am I missing some tunables that make HDFS
> do the right balance and replication?
> > Thanks,
> > Jeff
> Todd Lipcon
> Software Engineer, Cloudera