How do you max out a disk’s random accesses under Linux?

My original goal was to monitor disk_io_time in order to make sure our disks were not overloaded. However, how would I know what the limits of disk_io_time were, so I could correctly set the thresholds for the monitoring service?

I could max out random accesses and then observe the resulting disk_io_times. So I looked for a simple script to do that … and did not find any.

So here we go


help() {
   echo 'usage: read_random_disk_sectors BLOCK_DEVICE_NAME'
   echo '       read_random_disk_sectors --help'
   echo '    Will start reading random disk sectors one'
   echo '    after another. Useful for maxing out random'
   echo '    disk accesses'
   echo "    BLOCK_DEVICE_NAME should be something like 'sda'"
   echo "    You can run this command for 5 seconds by issuing"
   echo "    something like this:"
   echo "        sudo timeout -s INT 5s ./read_random_disk_sectors BLOCK_DEVICE_NAME"
   exit 1

[ "$1" == "--help" ] && help
if [ "$1" == "" ]; then
  ( echo 'ERROR: path to block device needs to be given'; echo ) >&2

set -e          # stop on error
set -u          # stop on undefined variable
set -o pipefail # stop part of pipeline failing

show_counter() {
  echo "I have done $counter reads"

trap show_counter INT

SECTORS=$( cat /sys/block/${DISK}/size )

while true; do
  SECTOR_TO_READ=$( shuf -i 0-${LAST_SECTOR} -n 1 )
  dd if=/dev/${DISK} of=/dev/null bs=512 skip=${SECTOR_TO_READ=} count=1 2>/dev/null
  counter=$(( $counter + 1 ))