> using mmap() is a no-brainer for fast file access and I'm surprised they hadn't used it before With mmap, you map a file into process memory and read and wri

> using mmap() is a no-brainer for fast file access and I'm surprised they hadn't used it before With mmap, you map a file into process memory and read and write to it as if it were memory. When you write to this memory there isn't really a way to indicate that an error occurred because there's no function call to return an error code. Instead, the process is sent a SIGBUS to indicate that something happened. This causes control to jump from the memory access into the signal handler. Normally this would be fine and you can handle the error there. But sqlite is a library. It lives in-process with the "real" application using it. What if the program had a signal handler there? You don't want to intercept signals that they need to receive, and you definitely don't want to mask errors that they are getting on their own mmapped files, and you don't know how to handle the errors that they should be seeing instead. And what if they have some crazy green-threading going on that you've now ripped control away from by entering the signal handler? In this particular case, sqlite's answer is to crash when that happens, taking down the entire host application with it. That's not okay for a lot of applications, and why it defaults to being turned off. And maybe this particular case does or doesn't apply to sqlite, but there are always cases that your application's domain-specific knowledge of how it uses disk and memory and CPU can out-perform the OS's more general optimisations. That's not the only issue, and the particular subtleties of mmap aren't important here. What's important is that when very smart people like the authors of sqlite choose to not do something, it's generally not as much as "no-brainer" as some know-it-all types would have you believe. It's easy to read something like Varnish's "always use mmap and leave it to the OS!" manifesto and think that you now automatically know better than all of those silly plebeians that just don't get it, but usually things are more nuanced than that. > I mean, its trivial to use if you know how to use fopen I seriously doubt that the difficulty of implementing it is a concern