Final answer:
The default threshold for flushing the largest Memtables in systems like Apache Cassandra is typically 40% of the heap memory. This process ensures data persistence by transferring data from volatile memory to disk storage.
Step-by-step explanation:
The default threshold of memory used for flushing of the largest Memtables in a database, particularly in systems like Apache Cassandra, is typically set at 40% of the heap memory. When the memory usage crosses this threshold, the system initiates a flush, which writes the contents of the Memtables to disk in a structure known as SSTable (Sorted Strings Table). This acts as a mechanism to control the memory usage and to ensure data durability, as it transfers data from volatile memory to a persistent storage.