#!/bin/bash
#
# Helper script to set a disk device queue_depth value.
# Usage:
# set_ncq <identifier> <value>
# <identifer> is part after /dev/, for example, sda
# <value> if omitted reports current setting, 1 means disable NCQ, otherwise sets queue_depth

# if no arguments, output usage
if [ $# -eq 0 ]; then
  echo "Usage: set_ncq <identifier> [ <value> ]"
  exit
fi

# if queue_depth file not present, then probably not SATA/SCSI device
if [ ! -e /sys/block/$1/device/queue_depth ]; then
#  echo "set_ncq: $1 does not support NCQ"
  exit
fi

# if only one argument, then report current queue_depth setting
if [ $# -eq 1 ]; then
  cat /sys/block/$1/device/queue_depth
  exit
fi

# if we're trying to disable NCQ and queue_depth is already 0 or 1, don't try to
# write queue_depth.  This avoids trying to write to a read-only queue_depth when device
# doesn't support NCQ, or when queue_depth can't be changed.
# Also supports devices for which 0 means 'disable NCQ' instead of 1 (are there any?).
if [ $2 -eq 1 -a `cat /sys/block/$1/device/queue_depth` -le 1 ]; then
#  echo "set_ncq: $1 NCQ already disabled"
  exit
fi

# set the desired queue_depth
echo "set_ncq: setting $1 queue_depth to $2"
echo $2 > /sys/block/$1/device/queue_depth