<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _add_mdisk(self, userid, disk, vdev):
 """Create one disk for userid NOTE: No read, write and multi password specified, and access mode default as 'MR'. """

        size = disk['size']
        fmt = disk.get('format', 'ext4')
        disk_pool = disk.get('disk_pool') or CONF.zvm.disk_pool
        [diskpool_type, diskpool_name] = disk_pool.split(':')

        if (diskpool_type.upper() == 'ECKD'):
            action = 'add3390'
        else:
            action = 'add9336'

        rd = ' '.join(['changevm', userid, action, diskpool_name,
                       vdev, size, '--mode MR'])
        if fmt and fmt != 'none':
            rd += (' --filesystem %s' % fmt.lower())

        action = "add mdisk to userid '%s'" % userid
        with zvmutils.log_and_reraise_smt_request_failed(action):
            self._request(rd)