No, the system actually is much larger than two nodes. But the number
of mappers used here tends to be fairly small (I suspect based on the
HBase regions being accessed but usually more than two), I'll try
turning off the combiner to see if that changes anything.
Thanks
- Adam
On 11/5/10 9:23 AM, Niels Basjes wrote:
Hi,
I don't know the answer (simply not enough information in your email)
but I'm willing to make a guess:
You are running on a system with two processing nodes?
If so then try removing the Combiner. The combiner is a performance
optimization and the whole processing should work without it.
Some times there is a design fault in the processing and the combiner
disrupts the processing.
HTH
Niels Basjes
2010/11/5 Adam Phelps <a...@opendns.com <mailto:a...@opendns.com>>
I've noticed an odd behavior with a map-reduce job I've written
which is reading data out of an HBase table. After a couple days of
poking at this I haven't been able to figure out the cause of the
problem, so I figured I'd ask on here.
(For reference I'm running with the cdh3b2 release)
The problem is that it seems that every line from the HBase table is
passed to the mappers twice, thus resulting in counts ending up as
exactly double what they should be.
I set up the job like this:
Scan scan = new Scan();
scan.addFamily(Bytes.toBytes(scanFamily));
TableMapReduceUtil.initTableMapperJob(table,
scan,
mapper,
Text.class,
LongWritable.class,
job);
job.setCombinerClass(LongSumReducer.class);
job.setReducerClass(reducer);
I've set up counters in the mapper to verify what is happening, so
that I know for certain that the mapper is being called twice with
the same bit of data. I've also confirmed (using the hbase shell)
that each entry appears only once in the table.
Is there a known bug along these lines? If not, does anyone have
any thoughts on what might be causing this or where I'd start
looking to diagnose?
Thanks
- Adam
--
Met vriendelijke groeten,
Niels Basjes
