#!/bin/bash

# adjust the following three values to your needs
DEV=sda
IDLEMIN=10
LOGFILE=/tmp/spindown.log

type echo | grep -qi built || echo "Warning: echo is not a built-in command"

STATFILE=/sys/block/$DEV/stat
PERIOD=5
let "CYCLES=$IDLEMIN*60/$PERIOD"
state=active

# keep 'sleep' executable in memory to prevent disk spin-up loading 'sleep'
sleep 100d &

rw=0
count=0
downtime=0
read start rest < /proc/uptime
start=${start%.*}
echo "" >> $LOGFILE
date "+%D %T - start monitoring of /dev/$DEV" >> $LOGFILE
while true ; do
  rw_old=$rw
  read rw < $STATFILE
  if [ "$rw" != "$rw_old" ] ; then
    if [ "$state" = "standby" ] ; then
      read now rest < /proc/uptime
      now=${now%.*}
      let diffs="$now-$lastacc"
      date -d "$diffs seconds ago" "+%D %T - idle" >> $LOGFILE
      let diffs="$now-$idle"
      let "downtime+=$diffs"
      let "percent=($downtime*100)/($now-$start)"
      date -d "$diffs seconds ago" "+%D %T - spin down" >> $LOGFILE
      let "h=$diffs/3600" "m=($diffs/60)%60" "s=$diffs%60"
      date "+%D %T - spin up after ${h}h ${m}m ${s}s (${percent}%% down)" >> $LOGFILE
    fi
    count=0
    state=active
    read lastacc rest < /proc/uptime
    lastacc=${lastacc%.*}
  else
    if [ "$state" = "active" ] ; then
      let "count=$count+1"
    fi
  fi
  if [ "$state" = "active" -a $count -ge $CYCLES ] ; then
    #hdparm -Y /dev/$DEV   # use this to actually spin down
    state=standby
    read idle rest < /proc/uptime
    idle=${idle%.*}
  fi
  sleep $PERIOD
done