Add new buffers_backend_fsync field to pg_stat_bgwriter.

This new field counts the number of times that a backend which writes a
buffer out to the OS must also fsync() it.  This happens when the
bgwriter fsync request queue is full, and is generally detrimental to
performance, so it's good to know when it's happening.  Along the way,
log a new message at level DEBUG1 whenever we fail to hand off an fsync,
so that the problem can also be seen in examination of log files
(if the logging level is cranked up high enough).

Greg Smith, with minor tweaks by me.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 6edb8bfd130..adab948e0dc 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -78,6 +78,7 @@ extern Datum pg_stat_get_bgwriter_buf_written_checkpoints(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_bgwriter_buf_written_clean(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_bgwriter_maxwritten_clean(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_buf_written_backend(PG_FUNCTION_ARGS);
+extern Datum pg_stat_get_buf_fsync_backend(PG_FUNCTION_ARGS);
 extern Datum pg_stat_get_buf_alloc(PG_FUNCTION_ARGS);
 
 extern Datum pg_stat_get_xact_numscans(PG_FUNCTION_ARGS);
@@ -1165,6 +1166,12 @@ pg_stat_get_buf_written_backend(PG_FUNCTION_ARGS)
 }
 
 Datum
+pg_stat_get_buf_fsync_backend(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_INT64(pgstat_fetch_global()->buf_fsync_backend);
+}
+
+Datum
 pg_stat_get_buf_alloc(PG_FUNCTION_ARGS)
 {
 	PG_RETURN_INT64(pgstat_fetch_global()->buf_alloc);