You can set the HDFS block size on Isilon, I believe the default is 64MB. You can view your current settings with: 'isi hdfs settings view' and change it with 'isi hdfs settings modify --default-block-size=128MB'.  I did some work last summer on Hadoop with Isilon and saw some definite advantages/efficiencies to using larger HDFS block sizes if your files are as large or larger than the selected HDFS block size.
Currently on my compute cluster block size is set to 128 MB.  I am not passing any custom split size etc.. in my MapReduce jobs, but I see the split size is 64MB. Does the ISILON has any block size implemented in OneFS hdfs process? Please explain. Thanks in advance. Ashok Beesam
ISILON OneFS 7.1.0.2 - hdfs block size ?