The "just mmap" pattern has also got portability issues if you might move the data file between systems which could have different endianness, pointer size, ali

The "just mmap" pattern has also got portability issues if you might move the data file between systems which could have different endianness, pointer size, alignment requirements, etc. (Serialization done properly avoids this because you can swap to a consistent host-independent on-the-wire representation as part of the process.) This is also less of an issue for the cases you suggest where you can regenerate the data from a primary source -- you can just treat the host-dependent data file as a cache that can be blown away if moving to a different machine.