It would also be good to see how many buffers were written by backend processes grouped by Buffer Access Strategy - to better distinguish evil backend writes from wanted backend writes.
Since all these writes are being cached by the operating system, which strategy writes them out isn't that useful to track. The only really "evil" type of writes are ones where the background writer doesn't absorb the fsync calls and the backends have to do that themselves. And as of V9.1, that is something you can distinguish in pg_stat_bgwriter (and it's also less likely to happen, too)
