reduce disk IO load by ionice when rsync

ionice -c3 rsync -avtz --progress src dst


-c class
The scheduling class. 0 for none, 1 for real time, 2 for best-
effort, 3 for idle.


Idle A program running with idle io priority will only get disk time
when no other program has asked for disk io for a defined grace
period. The impact of idle io processes on normal system activ‐
ity should be zero. This scheduling class does not take a prior‐
ity argument. Presently, this scheduling class is permitted for
an ordinary user (since kernel 2.6.25).