> ISTM aio_write only improves the picture if there's some magic in-kernel
> processing that makes this same kind of judgment as to when to issue the
> "ganged" write for real, and is able to do it on time because it's in
> the kernel. I haven't heard anything to make me think that that feature
> actually exists. AFAIK the kernel isn't much more enlightened about
> physical head positions than we are.
Can the magic be, that kaio directly writes from user space memory to the
disk ? Since in your case all transactions A-E want the same buffer written,
the memory (not it's content) will also be the same. This would automatically
write the latest possible version of our WAL buffer to disk.
The problem I can see offhand is how the kaio system can tell which transaction
can be safely notified of the write, or whether the programmer is actually responsible
for not changing the buffer until notified of completion ?
Andreas