Comment 5 for bug 1909861

AirDC++ actually has some kind of support for opening files O_DIRECT: https://github.com/airdcpp/airdcpp-windows/blob/b863d8626d95d0ee483572a5139f8f569b558c3f/airdcpp/airdcpp/File.cpp#L380-L394 (BUFFER_NONE isn't currently being used anywhere though when opening files)

FileReader::readCached currently uses the fadvise code path with POSIX_FADV_SEQUENTIAL.

I compiled AirDC++ with a modified FileReader that opens the file with BUFFER_NONE (O_DIRECT) and did a quick benchmark:

O_DIRECT: Hashing finished: 671 files (21.70 GiB) in 9 directories have been hashed in 4 minutes 10 seconds (88.62 MiB/s)
Normal caching (POSIX_FADV_SEQUENTIAL): Hashing finished: 671 files (21.70 GiB) in 9 directories have been hashed in 4 minutes 1 second (92.07 MiB/s)

Test builds for Linux: https://web-builds.airdcpp.net/develop/

So O_DIRECT is slower, but not by much. I still wonder whether the difference would be bigger with different disk setups (e.g. with network disks).