Well, this limits you to ~1 us granularity, but you are right that it's not the limiting factor. Apparently, according to https://stackoverflow.com/questions/1

Well, this limits you to ~1 us granularity, but you are right that it's not the limiting factor. Apparently, according to https://stackoverflow.com/questions/14392975/timestamp-accuracy-on-ext4-sub-millsecond https://stackoverflow.com/questions/14392975/timestamp-accur..., the ext4 driver just uses the cached kernel clock value without the counter correction that gives you a ns-precise value. One could perhaps have an LD_PRELOADED fsync (or whatever) that updates the mtime with clock_gettime() to store it in its full nanosecond precision glory but it's probably not worth the performance penalty. That wouldn't address the mmap issue of course...