Spindown USB disks on Snake OS NAS

One feature which I would like to see on Snake OS and it is not yet implemented is USB disk spindown. Few tips how to accomplish it can be found at http://www.nslu2-linux.org/wiki/FAQ/SpinDownUSBHarddisks. I tried sg_start from this howto and it just worked fine for my Transcend StoreJet 25.

The sg_start command is included in sg3_utils package. You can compile it with CNU SDK (you need to register to get SDK) in few steps:

• Download source tgz from http://sg.danny.cz/sg/sg3_utils.html#mozTocId479511
• Unpack
• Run configure script:
  CC=arm-linux-gcc ./configure --host arm-linux
• Compile:
  make

If everything went well you should find sg_start in src/.libs. You will also need libsgutils2.so.2.0.0 which can be found in libs/.libs directory. To install sg_start to NAS we have to copy both files (sg_start and libsgutils2.so.2.0.0) into NAS disk and rename libsgutils2.so.2.0.0 to libsgutils2.so.2.

Now we can try to spin-down disk on NAS by command:
LD_LIBRARY_PATH=./sg_start --stop /dev/sda

If the disk rotation is turned off we have succeeded.

The sg_start can only spin-down disk instantly. We have to create a simple script to spin-down disk after specified amount of time which it spent in idle:

#!/bin/sh
DISK_NAME="sda"
SPINDOWN_TIMEOUT=30
SPINDOWN_COMMAND="/usb/sda1/spindown/sg_start --stop /dev/$DISK_NAME"
SPINDOWN_LIB="/usb/sda1/spindown"
SPINDOWN_POLL_INTERVAL="60"
initial_sectors_read=`cat /proc/diskstats |grep "$DISK_NAME "| sed -e "s/[ ]*/ /g"|cut -d\  -f7`
initial_sectors_write=`cat /proc/diskstats |grep "$DISK_NAME "| sed -e "s/[ ]*/ /g"|cut -d\  -f11`
mins_idle=0
while [ 1 ]; do
  sleep $SPINDOWN_POLL_INTERVAL
  sectors_read=`cat /proc/diskstats |grep "$DISK_NAME "| sed -e "s/[ ]\+/ /g"|cut -d\  -f7`
  sectors_write=`cat /proc/diskstats |grep "$DISK_NAME "| sed -e "s/[ ]\+/ /g"|cut -d\  -f11`
  if [ $initial_sectors_read -ne $sectors_read ]; then
      initial_sectors_read=$sectors_read
      mins_idle=0
  elif [ $initial_sectors_write -ne $sectors_write ]; then
      initial_sectors_write=$sectors_write
      mins_idle=0
  else
      mins_idle=$(( $mins_idle+1 ))
  fi
  if [ $mins_idle -eq $SPINDOWN_TIMEOUT ]; then
      echo "Spin down disk by $SPINDOWN_COMMAND"
      export LD_LIBRARY_PATH=$SPINDOWN_LIB
      $SPINDOWN_COMMAND
  fi
  #echo $mins_idle
done

The script contains few variables which change it's behaviour (and you may need to change them):

• DISK_NAME - device identifier, sda is for first connected disk
  
• SPINDOWN_TIMEOUT - time in minutes which disk spend in idle is going to be spin-down
  
• SPINDOWN_COMMAND - command which spin down disk (the path is important in this variable)
• SPINDOWN_LIB - directory which contains libsgutils2.so.2
  
• SPINDOWN_POLL_INTERVAL - polling interval to reset idle counter or spin-down disk in seconds

The last thing is to do is to run script every time the NAS is started. I'm not sure how to do it right now by proper way. I use some workaround which alters inittab which I find wrong thus I'm not going to publish this solution. We may discuss it in comments.

You can get the script, library and sg_start binaries compiled for ARM here:

http://xhorak.fedorapeople.org/spindown.tar.gz