I/O sniffing

You can do I/O sniffing using the command blktrace. blktrace will show you every request that goes to the disk.

Example:

# blktrace -d /dev/sdg -o - | blkparse -i - 
  8,96   2        1     0.000000000  1236  G   N [kworker/2:4]
  8,96   2        2     0.000001308  1236  I   N 0 (00 ..) [kworker/2:4]
  8,96   2        3     0.000002781  1236  D   N 0 (00 ..) [kworker/2:4]

  8,96   7      106     0.373952974 11364  D   W 0 + 8 [kworker/7:2]
  8,96   7      107     0.374456639    47  C   W 0 + 8 [0]