# Tracing the loop device kernel module

The loop device kernel module can be traced with the *ftrace* framework of the
Linux kernel. Therefore, *ftrace* must be compiled into the Linux kernel and the
user space module *trace-cmd* is necessary.


## Creating the trace file

Each loop device specific operation can be traced with the following command
```bash
trace-cmd record -p function_graph --module loop -o [trace file] [cmd]
```
where `[trace file]` is the created trace file and `[cmd]` is one of the
following loop device operation:
  * add loop device: `losetup -f test.img`
  * read from loop device: `dd if=/dev/loop0 bs=512 count=1 of=tmp.img`
  * write to loop device: `dd of=/dev/loop0 bs=512 count=1 if=tmp.img`
  * remove loop device: `losetup -D`


## Viewing the trace file

The created trace file can be viewed on the command line with the following
command:
```bash
trace-cmd report -i [trace file]
```
where `[trace file]` is the already created trace file.