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MapReduce >> mail # user >> Question on HDFS_BYTES_READ and HDFS_BYTES_WRITTEN


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Question on HDFS_BYTES_READ and HDFS_BYTES_WRITTEN
I have an iterative MapReduce job that I run over 35 GB of data repeatedly.
The output of the first job is the input to the second one and it goes on
like that until convergence.

I am seeing a strange behavior with the program run time. The first
iteration takes 4 minutes to run and here is how the counters look:

HDFS_BYTES_READ                    34,860,867,377
HDFS_BYTES_WRITTEN               45,573,255,806

The second iteration takes 15 minutes and here is how the counters look in
this case:

HDFS_BYTES_READ                   144,563,459,448
HDFS_BYTES_WRITTEN               49,779,966,388

I cannot explain these numbers because the first iteration - to begin with
- should only generate approximately 35 GB of output. When I check the
output size using

hadoop fs -dus

I can confirm that it is indeed 35 GB. But for some reason
HDFS_BYTES_WRITTEN shows 45 GB. Then the input to the second iteration
should be 35 GB (or even 45GB considering the counter value)
but HDFS_BYTES_READ shows 144 GB.

All following iterations produce similar counter values to the second one
and they roughly take 15 min each. My dfs replication factor is set to 1
and there is no compression turned on. All input and outputs are in
SequenceFile format. The initial input is a sequence file that I generated
locally using SequenceFile.Writer but I use the default values and as far
as I know compression should be turned off.  Am I wrong?

Thanks in advance.