When a drive has lots of activity for seemingly no reason, what tools are available to troubleshoot? Here are a few possibilities.
Debian provides a tool called iotop, via the package iotop, which, by default, provides a 'top'-like experience. A different command line experience can be achieved through something like (which every 10secs prints a list of processes that read/wrote to disk and the amount of IO bandwidth used):
# iotop -o -b -d 10 Total DISK READ : 0.00 B/s | Total DISK WRITE : 0.00 B/s Actual DISK READ: 0.00 B/s | Actual DISK WRITE: 0.00 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND Total DISK READ : 0.00 B/s | Total DISK WRITE : 2041.86 B/s Actual DISK READ: 0.00 B/s | Actual DISK WRITE: 1837.68 B/s TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND 19967 be/4 root 0.00 B/s 2041.86 B/s 0.00 % 0.00 % conntrackd -C /etc/conntrackd/conntrackd.conf 14023 be/4 root 0.00 B/s 0.00 B/s 0.00 % 0.00 % perl /usr/sbin/x2gocleansessions
More use cases are available at How to find which process is regularly writing to disk?
General statistics, which could be coupled with a 'wait', or in this case it refreshes itself with -k:
# iostat -xk 2 /dev/sdb Linux 4.14.0-0.bpo.3-amd64 (host01.ny1) 05/26/18 _x86_64_ (16 CPU) avg-cpu: %user %nice %system %iowait %steal %idle 0.31 0.00 0.66 0.09 0.00 98.94 Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util sdb 0.00 1.78 0.24 6.80 98.41 432.03 150.76 0.60 85.97 114.91 84.95 2.13 1.50