[caf]

Readahead is still useful for large files read sequentially once, and that needs to be buffered. Such programs should use posix_fadvise().

[paulsutter]

you can readahead as far as you like with unbuffered io

[caf]

If you are reading unbuffered (ie. O_DIRECT) then you are reading directly into the memory block the user supplied, so you cannot read ahead - there's nowhere to put the extra data.

[paulsutter]

Of course you can read ahead using multiple buffers, you can issue as many reads as you want concurrently

[caf]

I think it is pretty clear I was referring to kernel-mediated readahead. Sure, you can achieve the same thing in userspace using async IO or threads.

[paulsutter]

I think it’s pretty clear that I was referring to reading large files using direct io, and your comment was totally irrelevant

[caf]

Reading large files using direct IO defeats kernel read-ahead, which means you have to take on the complexity of reimplementing it in userspace, or the performance hit of not having it.

This is a good reason for programs to use buffered IO even when they are reading a large file once, so yes my comment was entirely on point.