It seems that the blktrace command is not working at all.
# blktrace -d $DEV
FAILED to start thread on CPU 156: 22/Invalid argument
FAILED to start thread on CPU 157: 22/Invalid argument
FAILED to start thread on CPU 158: 22/Invalid argument
FAILED to start thread on CPU 159: 22/Invalid argument
# echo $DEV
/dev/loop0
It seems that the blktrace command is not working at all.
# blktrace -d $DEV
FAILED to start thread on CPU 156: 22/Invalid argument
FAILED to start thread on CPU 157: 22/Invalid argument
FAILED to start thread on CPU 158: 22/Invalid argument
FAILED to start thread on CPU 159: 22/Invalid argument
# echo $DEV
/dev/loop0
So the output is empty.