source: vbox/trunk/src/VBox/Devices/PC/ipxe/contrib/vm/cow

#!/bin/sh

set -e

imgloop=
tmpfile=
tmploop=
dmname=
cowlink=

function cleanup () {
    set +e
    [ -n "$cowlink" ] && rm $cowlink
    [ -n "$dmname" ] && dmsetup remove $dmname
    [ -n "$tmploop" ] && losetup -d $tmploop
    [ -n "$tmpfile" ] && rm $tmpfile
    [ -n "$imgloop" ] && losetup -d $imgloop
}

trap cleanup EXIT

imgfile=$1 ; shift
command=$1 ; shift
if [ -z "$imgfile" -o -z "$command" ] ; then
    echo Syntax: $0 /path/to/image/file command [args..]
    exit 1
fi

# Set up image loop device
x=`losetup -f` ; losetup -r $x $imgfile ; imgloop=$x

# Create temporary file and set up temporary loop device
tmpfile=`mktemp $imgfile.XXXXXXXXXX`
truncate -r $imgfile $tmpfile
x=`losetup -f` ; losetup $x $tmpfile ; tmploop=$x

# Create snapshot device
imgsize=`blockdev --getsz $imgloop`
x=`basename $imgfile` ; echo 0 $imgsize snapshot $imgloop $tmploop N 16 | \
    dmsetup create $x ; dmname=$x
chown --reference=$imgfile /dev/mapper/$dmname
chmod --reference=$imgfile /dev/mapper/$dmname

# Create symlink
x=$imgfile.cow ; ln -s /dev/mapper/$dmname $x ; cowlink=$x

# Wait until killed
echo "Created $cowlink"
$command "$@" $cowlink