def METHOD_NAME(vm_name, device_source, device_target):
    """
    Attach a disk with an iothread value of 2.
    """
    disk_attach_success = params.get("virt_disk_attach_success")
    attach_option = params.get("disk_attach_option_io_2")
    ret = virsh.attach_disk(vm_name, device_source,
                            device_target,
                            attach_option, debug=True)
    disk_attach_error = False
    libvirt.check_exit_status(ret, disk_attach_error)
    libvirt.check_result(ret, expected_match=disk_attach_success)