[astrange]

If you're reading sequentially this shouldn't be a problem because the VM system can pick up hints, or you can use madvise.

If you're reading randomly this is true and you want some kind of async I/O or multiple read operation.

mmap is also dangerous because there's no good way to return errors if the I/O fails, like if the file is resized or is on an external drive.

[jandrewrogers]

Even if you use madvise() for a large sequential read, the kernel will often restrict its behavior to something suboptimal with respect to performance on modern hardware.