$ head -c 8M < /dev/urandom > data_unbuf.txt
$ head -c 8M < /dev/urandom > data_buf.txt
$ # Write data to disk and flush caches completely
$ sync && echo 3 | sudo tee /proc/sys/vm/drop_caches
3
$ cargo script posix_fadvise_test.rs
Compiling posix_fadvise_test v0.1.0 (file:///home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286)
Unadvised result: HashResult { hash: [0, 42, 55, 246, 0, 0, 18, 138, 140, 13, 45, 185, 239, 203, 220, 112], time: Duration { secs: 0, nanos: 26310368 } }
Advised result: HashResult { hash: [225, 161, 142, 219, 14, 193, 119, 103, 106, 26, 140, 250, 238, 212, 83, 168], time: Duration { secs: 0, nanos: 3485998 } }
$ # Comment out the call to advise_willneed(), flush caches again, and re-run
$ sync && echo 3 | sudo tee /proc/sys/vm/drop_caches
3
$ cargo script posix_fadvise_test.rs
Compiling posix_fadvise_test v0.1.0 (file:///home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286)
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:42:1: 55:2 warning: function is never used: `advise_willneed`, #[warn(dead_code)] on by default
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:42 fn advise_willneed(file: &File) {
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:43     let len = file.metadata().unwrap().len();
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:44
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:45     unsafe {
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:46         assert_eq!(
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:47             libc::posix_fadvise(
                                                                                                   ...
Unadvised result: HashResult { hash: [0, 42, 55, 246, 0, 0, 18, 138, 140, 13, 45, 185, 239, 203, 220, 112], time: Duration { secs: 0, nanos: 27309768 } }
Advised result: HashResult { hash: [225, 161, 142, 219, 14, 193, 119, 103, 106, 26, 140, 250, 238, 212, 83, 168], time: Duration { secs: 0, nanos: 19609183 } }
$ # Notice the huge increase in the duration for the advised result