'Add a retry entry for the selected volume backend.
In the event that the request gets re-scheduled, this entry will signal
that the given backend has already been tried.'

def _add_retry_backend(self, filter_properties, backend):

    retry = filter_properties.get('retry', None)
    if (not retry):
        return
    for key in ('hosts', 'backends'):
        backends = retry.get(key)
        if (backends is not None):
            backends.append(backend)
