Backend fsyncs are theoretically still possible after the fsync
request queue compaction patch (which was subsequently back-patched to
all supported release branches). However, I'm reasonably confident
that that patch was so effective as to make a backend fsync all but
impossible. As such, it seems like the buffers_backend_fsync column in
the pg_stat_bgwriter view is more or less obsolete.
I suggest removing it for 9.5, and instead logging individual
occurrences of backend fsync requests within ForwardFsyncRequest(). It
seems fair to treat that as an anomaly to draw particular attention
to.
--
Peter Geoghegan