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From Gabriel Reid <gabriel.r...@gmail.com>
Subject Issues running a large MapReduce job over a complete HBase table
Date Mon, 06 Dec 2010 08:02:56 GMT

We're currently running into issues with running a MapReduce job over
a complete HBase table - we can't seem to find a balance between
having dfs.datanode.max.xcievers set too low (and getting
"xceiverCount X exceeds the limit of concurrent xcievers") and getting
OutOfMemoryErrors on datanodes.

When trying to run a MapReduce job on the complete table we inevitably
get one of the two above errors eventually -- using a more restrictive
Scan with a startRow and stopRow for the job runs without problems.

An important note is that the table that is being scanned has a large
disparity in the size of the values being stored -- one column family
contains values that are all generally around 256 kB in size, while
the other column families in the table contain values that are closer
to 256 bytes. The hbase.hregion.max.filesize setting is still at the
default (256 MB), meaning that we have HFiles for the big column that
are around 256 MB, and HFiles for the other columns that are around
256 kB. The dfs.datanode.max.xcievers setting is currently at 2048,
and this is running a 5-node cluster.

The table in question has about 7 million rows, and we're using
Cloudera CDH3 (HBase 0.89.20100924 and Hadoop 0.20.2).

As far as I have been able to discover, the correct thing to do (or to
have done) is to set the hbase.hregion.max.filesize to a larger value
to have a smaller number of rows, which as I understand would probably
solve the issue here.

My questions are:
1. Is my analysis about having a larger hbase.hregion.max.filesize correct?
2. Is there something else that we can do to resolve this?
3. Am I correct in assuming that the best way to resolve this now is
to make the hbase.hregion.max.filesize setting larger, and then use
the org.apache.hadoop.hbase.util.Merge tool as discussed at
http://osdir.com/ml/general/2010-12/msg00534.html ?

Any help on this would be greatly appreciated.



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