This has been visibly a forgotten spot in the first implementation of wait events for I/O added by 249cf07, and what has been missing is a fsync call for WAL segments which is a wrapper reacting on the value of GUC wal_sync_method.
c55de5e512 Add wait event for fsync of WAL segments
doc/src/sgml/monitoring.sgml | 4 ++++
src/backend/access/transam/xlog.c | 2 ++
src/backend/postmaster/pgstat.c | 3 +++
src/include/pgstat.h | 1 +
4 files changed, 10 insertions(+)