Why does splitting $PGDATA and xlog yield a performance benefit?

Howdy All,

For a very long time I've held the belief that splitting PGDATA and xlog on linux systems fairly universally gives a
decentperformance benefit for many common workloads. 
(i've seen up to 20% personally).

I was under the impression that this had to do with regular fsync()'s from the WAL
interfearing with and over-reaching writing out the filesystem buffers.

Basically, I think i was conflating fsync() with sync().

So if it's not that, then that just leaves bandwith (ignoring all of the other best practice reasons for reliablity,
etc.).So, in theory if you're not swamping your disk I/O then you won't really benefit from relocating your XLOGs. 

However, I know from experience that's not entirely true, (although it's not always easy to measure all aspects of your
I/Obandwith). 

Am I missing something?

Thanks