Earlier this week I upgraded to CDH 5.4.5 from 5.4.4. Since then I can see that the datanode process is constantly reading 2M/s per drive in each host (with a number of writes maybe one order of magnitude smaller), but there is no corresponding HDFS I/O activity (just the usual activity, ~40k/s)  It seems as if nobody is actually reading anything, but the HDFS process is doing something on its own.   Any ideas about what could be causing this? How could I find out / diagnose what's happening?   Thanks! ... View more