disk attack: support inject write/read payload for specified duration

[WangXiangUSTC]

For example, I want to inject a write payload while an experiment, and the experiment need to run for a long time. I need to write a shell script like the below:

#!/bin/bash

for((i=1;i<=100;i++));
do
    ./chaosd attack disk add-payload write -p /chaosd_data -n 2 --size 100G
    rm /home/root/chaosd_data
done

I can't set the size with a large value, because the storage capacity is limited.

Maybe we should add duration config for disk attack, and chaosd will write and delete automatically.