If an I/O error happens with read()/write(), you get back an error code, which SQLite can deal with and pass back up to the application, perhaps accompanied by a reasonable error message. But if you get an I/O error with mmap, you get a signal. SQLite itself ought not be setting signal handlers, as that is the domain of the application and SQLite is just a lowly library. And even if SQLite could set signal handlers, it would be difficult to associate a signal with a particular I/O operation. So there isn't a good way to deal with I/O errors when using mmap(). With mmap(), you just have to assume that the filesystem/mass-storage works flawlessly and never runs out of space.

SQLite can use mmap(). That is a tested and supported capability. But we don't advocate it because of the inability to precisely identify I/O errors and report them back up into the application.

I know that the spirit of HN will strike me down for this, but sqlite is not a "sophisticated system". It assumes the hardware is lawful neutral. Real hardware is chaotic. Sqlite has a good reputation because it is very easy to use. In fact this is the same reason programmers like mmap: it is a hell of a shortcut.