Could you rebase? Marked as waiting-on-author for now.
Attached is the rebased/rewritten version of the pg_stat_buffers patch which uses the cumulative stats system instead of stats collector.
I've moved to the model of backend-local pending stats which get accumulated into shared memory by pgstat_report_stat().
It is worth noting that, with this method, other backends will no longer have access to each other's individual IO operation statistics. An argument could be made to keep the statistics in each backend in PgBackendStatus before accumulating them to the cumulative stats system so that they can be accessed at the per-backend level of detail.
There are two TODOs related to when pgstat_report_io_ops() should be called. pgstat_report_io_ops() is meant for backends that will not commonly call pgstat_report_stat(). I was unsure if it made sense for BootstrapModeMain() to explicitly call pgstat_report_io_ops() and if auto vacuum worker should call it explicitly and, if so, if it was the right location to call it after do_autovacuum().
Archiver and syslogger do not increment or report IO operations.
I did not change pg_stat_bgwriter fields to derive from the IO operations statistics structures since the reset targets differ.
Also, I added one test, but I'm not sure if it will be flakey. It tests that the "writes" for checkpointer are tracked when data is inserted into a table and then CHECKPOINT is explicitly invoked directly after. I don't know if this will have a problem if the checkpointer is busy and somehow the backend which dirtied the buffer is forced to write out its own buffer, causing the test to potentially fail (even if the checkpointer is doing other writes [causing it to be busy], it may not do them in between the INSERT and the SELECT from pg_stat_buffers).
I am wondering how to add a non-flakey test. For regular backends, I couldn't think of a way to suspend checkpointer to make them do their own writes and fsyncs in the context of a regression or isolation test. In fact for many of the dirty buffers it seems like it will be difficult to keep bgwriter, checkpointer, and regular backends from competing and sometimes causing test failures.
