iotop does for I/O usage what top does for CPU usage. It watches I/O usage information output by the Linux kernel (requires 2.6.20 or later) and displays a table of current I/O usage by processes on the system. Handy for answering the question "Why is my disk churning so much?".
Install iotop in debian
#apt-get install iotop
This will complete the installation.
type iotop in your terminal and you will get a regularly updated list of processes and the amount of I/O they are currently using: reads and writesin bytes per second, and percentage of time spent swapping and waiting for I/O.
iotop needs the CONFIG_TASK_DELAY_ACCT and CONFIG_TASK_IO_ACCOUNTING options enabled in the kernel; in Debian stock kernels, that’s already the case.