Why is a persistent layer still necessary?

The persistent layer in a database management system is essential for maintaining data integrity and durability. This layer serves as a permanent storage solution, ensuring that data remains accessible even after it has been removed from the memory for various reasons such as efficiency or memory constraints.

When data is unloaded from memory, it is crucial to have a dedicated place where this information can be safely stored for future retrieval, which highlights the primary role of the persistent layer. This function not only protects the data during unforeseen system failures but also facilitates optimization of memory usage by offloading data that is not frequently accessed yet still needs to be retained.

The other options focus on different aspects of data management, such as delta stores, which track changes to records, or recovery processes in case of power failures. While these functions are important, they do not encapsulate the fundamental reason for the existence of the persistent layer as effectively as the need to store data that has been unloaded from memory. This aspect underscores the critical role of the persistent layer in ensuring long-term data availability and reliability within the system.