I can’t explain well what’s the differencies between Hadoop and GridGain, but there is one Wiki Page on Hadoop that’s explain it
(http://wiki.apache.org/hadoop/HadoopVsGridGain) and the discussion between co founder of GridGain and Owen (http://gridgain.blogspot.com/2008/05/gridgain-vs-hadoop-continued.html)
From the last article, i found the JIRA Hadoop ticket (https://issues.apache.org/jira/browse/HADOOP-2536), this is where the joy come,
with this now Hadoop have support for running MapReduce jobs over data residing in a MySQL table (and another database may be in the future …).
On 0.19 Database as an input for MapReduce already implemented, you can see the Hadoop API docs for more detal (org.apache.hadoop.mapred.lib.db)
Happy Hadoopying ..!