Sharing a solid-state drive (SSD) among multiple tenants has become a common way to improve the storage resource utilization and reduce the operating cost. This is driven by the high-density flash technology development, highly parallel storage architecture inside SSDs, and the full support of interface techniques. However, multiple tenants contend for the limited storage resources including the time resource and space resource of the SSD and interfere with each other. How to guarantee the quality of service (QoS) of multi-tenant SSDs has been a hot research topic in recent years. In this paper, we analyze and summarize the current state of research about the QoS technologies of multi-tenant SSDs. Firstly, we analyze the three QoS problems of multi-tenant SSDs: performance interference among tenants, performance unfairness among tenants, and loss of overall performance. Then, we present a comprehensive classification and introduction to existing work for improving the QoS of multi-tenant SSDs from three types of objectives, i.e., performance isolation, fairness guarantee, and overall performance optimization, from two dimensions of time resource and space resource. The technical evolution line of existing work is also sorted out. Finally, we conclude the existing techniques for QoS assurance of multi-tenant SSDs and discuss potential future research directions.