Well following is the outputs of command iostat:
Linux 4.13.0-45-generic (Monster) 	2018年09月12日 	_x86_64_	(48 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          11.63    0.00    2.17    1.75    0.00   84.44

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda             130.83       623.48       729.04   75656481   88466496
sdb              27.32      1358.62        26.68  164862720    3237276
sdc            1978.66     95973.51       221.50 11645990012   26878608
sdd               0.68        11.31       245.21    1371866   29754852

During which two imagenet training tasks are initiating their dataloaders, and the data is stored on /dev/sdc1.