Writes to mmap-ed regions aren't very reliable: besides the synchronization issue there's also the question of what happens on an I/O error or ENOSPC. Usually your program just gets a SIGBUS and crashes.
IIRC LMDB uses regular syscalls to update the mapping, and use mmap only for read.
Correct, at least for the original design of LMDB. We later added an option to write thru the mmap as well, at the request of some other users. Since LMDB is quite careful about writing and use of fsync/msync, reliability generally isn't an issue, but yes, hard errors on the disk and such can crash your program when using a writable mmap. Also, in the default mode of read-only mmap, the DB is incorruptible, stray pointer writes will crash the program instead of corrupting the DB. With the writable mmap you obviously have no such protection. We recommend developing your app with the default read-only mode, and only enabling the writable mmap after your app code is well tested.
