What type of storage is required for unlimited retention of data?

The requirement for unlimited retention of data is well fulfilled by cold storage, which is designed specifically for long-term data archiving and retention. Cold storage services are typically optimized for storing large volumes of data that are not accessed frequently but need to be retained for compliance or regulatory reasons. This type of storage is cost-effective and allows for an extensive retention period, making it ideal for data that is not regularly used but must be kept indefinitely.

In contrast, warm storage is meant for data that is accessed less frequently than hot storage but more frequently than cold storage, and it does not necessarily support unlimited retention effectively. Cosmos DB, while a powerful database service providing numerous features for managing data, does not focus on the aspects of cost-effectiveness for long-term archival storage and is better suited for applications requiring rapid access and scalability. Blob storage is also a strong candidate for data storage but is more geared towards use cases that require frequent access compared to data that is mostly for archival purposes.

Overall, the unique nature of cold storage directly addresses the need for unlimited data retention in a cost-effective manner, making it the most appropriate choice in scenarios where data must be stored for long periods without regular access.