Caching

Memory Mapped Storage Engine

This is the current storage engine for MongoDB, and it uses memory-mapped files for all disk I/O.  Using this strategy, the operating system's virtual memory manager is in charge of caching.  This has several implications:

• There is no redundancy between file system cache and database cache: they are one and the same.
• MongoDB can use all free memory on the server for cache space automatically without any configuration of a cache size.
• Virtual memory size and resident size will appear to be very large for the mongod process.  This is benign: virtual memory space will be just larger than the size of the datafiles open and mapped; resident size will vary depending on the amount of memory not used by other processes on the machine.
• Caching behavior (such as LRU'ing out of pages, and laziness of page writes) is controlled by the operating system: quality of the VMM implementation will vary by OS.