def ModifyNodeStorageUnits(r, node, storage_type, name, allocatable=None):
    """
    Modifies parameters of storage units on the node.

    @type node: str
    @param node: node whose storage units to modify
    @type storage_type: str
    @param storage_type: storage type whose units to modify
    @type name: str
    @param name: name of the storage unit
    @type allocatable: bool or None
    @param allocatable: Whether to set the "allocatable" flag on the storage
                                            unit (None=no modification, True=set, False=unset)

    @rtype: int
    @return: job id
    """
    query = {'storage_type': storage_type, 'name': name}
    if allocatable is not None:
        query['allocatable'] = allocatable
    return r.request('put', '/2/nodes/%s/storage/modify' % node, query=query)
