Function read_back 

pub fn read_back(
    path: &Path,
    buf: &mut AlignedBuf,
    try_direct: bool,
) -> Result<(u64, f64, ReadHow), String>

Read path back sequentially into buf, past the page cache: O_DIRECT when try_direct and the filesystem takes it, else drop the cached pages (fadvise DONTNEED, after the caller’s fsync they are clean) and read buffered. Returns (bytes, seconds, how); the clock starts after the open so the number is the transfer, not the open.