The current asynchronous I/O (aio) implementation works by forking a background thread inside the kernel to do the I/O synchronously. This is a starting point, but one thread limits the amount of potential parallelism, and adding more threads falls down badly when applications want to have large numbers of requests outstanding at once.
Furthermore, the existing code doesn't even work in some cases; this is not well documented but there have been scattered reports of problems that nobody had time to look into in detail.
In order to make asynchronous I/O work well, the I/O path needs to be revised, particularly in the kernel's file system interface, so that all I/O operations are asynchronous requests by default. It is easy for high-level code to wait synchronously for lower-level asynchronous requests to complete; it is much more problematic for an asynchronous request to call into code that expects to be synchronous.
Theproject, which also requires substantial revisions to the I/O path, naturally combines with this project.