If personal data has not been collected from the data subject, the person responsible must inform the data subject. The information hast to contain e.g. the name and contact details of the person responsible for the purpose of processing the personal data and the respective category are disclosed. Get here more information.
Art. 14 para. 5 lit. B GDPR provides for an exception to the obligation to provide information if the provision of information is classified as impossible or would require a disproportionate effort. The number of data subjects and the age of the data play an important role. According to Article 29 of the Data Protection Working Party, the controller has to balance his information and the information interests of the data subject. The result must be documented. However, if the legal basis for an exception to the information obligation exists, appropriate measures must be taken to safeguard the rights, freedoms and interests of the data subject.