To monitor i/o on a disk, iostat can be used. It is installed with:
apt install sysstat
Example output which shows since time of boot:
# iostat Linux 4.12.0-2-amd64 (np8157) 09/24/2017 _x86_64_ (8 CPU) avg-cpu: %user %nice %system %iowait %steal %idle 9.73 0.01 0.74 0.07 0.00 89.45 Device tps kB_read/s kB_wrtn/s kB_read kB_wrtn nvme0n1 5.23 16.69 421.26 1327937 33507977 sda 2.32 852.50 14.36 67809624 1142456 sdb 1.22 523.62 14.36 41649436 1142456 sdf 7.60 5.60 1119.00 445440 89007204
Stats in a two second interval:
iostat -h -d 2 Linux 4.12.0-2-amd64 (np8157) 09/24/17 _x86_64_ (8 CPU) Device tps kB_read/s kB_wrtn/s kB_read kB_wrtn nvme0n1 5.72 16.1k 462.6k 1.3G 37.7G sda 10.17 4.7M 14.5k 395.2G 1.2G sdb 9.22 4.4M 14.5k 370.4G 1.2G sdf 7.48 5.2k 1.1M 436.1M 88.1G