During 2002, Linus Torvalds had proposed another implementation for O_DIRECT (External link). During that time, O_DIRECT was not as performant, and it showed up to a 55% performance hit vs no O_DIRECT. Linus attributed this problem to the fact that O_DIRECT needed to be asynchronous and had to do read-ahead.

What Linus had proposed when doing an O_DIRECT read into a buffer is to divide this process in two phases:

  1. Allocate the pages, and start the I/O operation asynchronously.
  2. mmap the file with a MAP_UNCACHED flag, causing read-faults to “steal” the page from the page cache and making it private to the mapping on the page faults.

And any write() operation would be the other way around:

  1. Take the pages in the memory area and move them to the page cache, removing the page from the page table (and only copying it if pages already exist).
  2. Make the I/O operation to disk.

With this approach, the kernel would not have to make a copy of the buffer that the process is trying to flush to disk into its own cache (to immediately send to disk), but would just take ownership of this allocated buffer from the process, and be able to do any operations it needs with it (including flushing it to disk).

While a clever implementation that would make O_DIRECT operations more performant, the only change that was made to the design was the asynchronous processing of O_DIRECT. Because most of the database systems (and other complicated programs) were already using the POSIX-inspired API, the only update that was made to the design of this API was the underlying processing for these calls.