'Retrieve all the DBInstances in your account.
:type instance_id: str
:param instance_id: DB Instance identifier.  If supplied, only
information this instance will be returned.
Otherwise, info about all DB Instances will
be returned.
:type max_records: int
:param max_records: The maximum number of records to be returned.
If more results are available, a MoreToken will
be returned in the response that can be used to
retrieve additional records.  Default is 100.
:type marker: str
:param marker: The marker provided by a previous request.
:rtype: list
:return: A list of :class:`boto.rds.dbinstance.DBInstance`'

def get_all_dbinstances(self, instance_id=None, max_records=None, marker=None):

    params = {}
    if instance_id:
        params['DBInstanceIdentifier'] = instance_id
    if max_records:
        params['MaxRecords'] = max_records
    if marker:
        params['Marker'] = marker
    return self.get_list('DescribeDBInstances', params, [('DBInstance', DBInstance)])
