Add WAL data to backend statistics

This commit adds per-backend WAL statistics, providing the same
information as pg_stat_wal, except that it is now possible to know how
much WAL activity is happening in each backend rather than an overall
aggregate of all the activity.  Like pg_stat_wal, the implementation
relies on pgWalUsage, tracking the difference of activity between two
reports to pgstats.

This data can be retrieved with a new system function called
pg_stat_get_backend_wal(), that returns one tuple based on the PID
provided in input.  Like pg_stat_get_backend_io(), this is useful when
joined with pg_stat_activity to get a live picture of the WAL generated
for each running backend, showing how the activity is [un]balanced.

pgstat_flush_backend() gains a new flag value, able to control the flush
of the WAL stats.

This commit relies mostly on the infrastructure provided by
9aea73fc61d4, that has introduced backend statistics.

Bump catalog version.  A bump of PGSTAT_FILE_FORMAT_ID is not required,
as backend stats do not persist on disk.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/Z3zqc4o09dM/Ezyz@ip-10-97-1-34.eu-west-3.compute.internal

1 files changed, 84 insertions, 3 deletions

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 6efbb650aa8..da4c7edd772 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -39,6 +39,14 @@
 static PgStat_BackendPending PendingBackendStats = {0};
 
 /*
+ * WAL usage counters saved from pgWalUsage at the previous call to
+ * pgstat_report_wal().  This is used to calculate how much WAL usage
+ * happens between pgstat_report_wal() calls, by subtracting the previous
+ * counters from the current ones.
+ */
+static WalUsage prevBackendWalUsage;
+
+/*
  * Utility routines to report I/O stats for backends, kept here to avoid
  * exposing PendingBackendStats to the outside world.
  */
@@ -185,6 +193,57 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
 }
 
 /*
+ * To determine whether WAL usage happened.
+ */
+static inline bool
+pgstat_backend_wal_have_pending(void)
+{
+	return (pgWalUsage.wal_records != prevBackendWalUsage.wal_records);
+}
+
+/*
+ * Flush out locally pending backend WAL statistics.  Locking is managed
+ * by the caller.
+ */
+static void
+pgstat_flush_backend_entry_wal(PgStat_EntryRef *entry_ref)
+{
+	PgStatShared_Backend *shbackendent;
+	PgStat_WalCounters *bktype_shstats;
+	WalUsage	wal_usage_diff = {0};
+
+	/*
+	 * This function can be called even if nothing at all has happened for WAL
+	 * statistics.  In this case, avoid unnecessarily modifying the stats
+	 * entry.
+	 */
+	if (!pgstat_backend_wal_have_pending())
+		return;
+
+	shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
+	bktype_shstats = &shbackendent->stats.wal_counters;
+
+	/*
+	 * Calculate how much WAL usage counters were increased by subtracting the
+	 * previous counters from the current ones.
+	 */
+	WalUsageAccumDiff(&wal_usage_diff, &pgWalUsage, &prevBackendWalUsage);
+
+#define WALSTAT_ACC(fld, var_to_add) \
+	(bktype_shstats->fld += var_to_add.fld)
+	WALSTAT_ACC(wal_buffers_full, wal_usage_diff);
+	WALSTAT_ACC(wal_records, wal_usage_diff);
+	WALSTAT_ACC(wal_fpi, wal_usage_diff);
+	WALSTAT_ACC(wal_bytes, wal_usage_diff);
+#undef WALSTAT_ACC
+
+	/*
+	 * Save the current counters for the subsequent calculation of WAL usage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
+}
+
+/*
  * Flush out locally pending backend statistics
  *
  * "flags" parameter controls which statistics to flush.  Returns true
@@ -194,12 +253,23 @@ bool
 pgstat_flush_backend(bool nowait, bits32 flags)
 {
 	PgStat_EntryRef *entry_ref;
+	bool		has_pending_data = false;
 
 	if (!pgstat_tracks_backend_bktype(MyBackendType))
 		return false;
 
-	if (pg_memory_is_all_zeros(&PendingBackendStats,
-							   sizeof(struct PgStat_BackendPending)))
+	/* Some IO data pending? */
+	if ((flags & PGSTAT_BACKEND_FLUSH_IO) &&
+		!pg_memory_is_all_zeros(&PendingBackendStats.pending_io,
+								sizeof(struct PgStat_PendingIO)))
+		has_pending_data = true;
+
+	/* Some WAL data pending? */
+	if ((flags & PGSTAT_BACKEND_FLUSH_WAL) &&
+		pgstat_backend_wal_have_pending())
+		has_pending_data = true;
+
+	if (!has_pending_data)
 		return false;
 
 	entry_ref = pgstat_get_entry_ref_locked(PGSTAT_KIND_BACKEND, InvalidOid,
@@ -211,6 +281,9 @@ pgstat_flush_backend(bool nowait, bits32 flags)
 	if (flags & PGSTAT_BACKEND_FLUSH_IO)
 		pgstat_flush_backend_entry_io(entry_ref);
 
+	if (flags & PGSTAT_BACKEND_FLUSH_WAL)
+		pgstat_flush_backend_entry_wal(entry_ref);
+
 	pgstat_unlock_entry(entry_ref);
 
 	return false;
@@ -226,7 +299,8 @@ pgstat_backend_have_pending_cb(void)
 		return false;
 
 	return (!pg_memory_is_all_zeros(&PendingBackendStats,
-									sizeof(struct PgStat_BackendPending)));
+									sizeof(struct PgStat_BackendPending)) ||
+			pgstat_backend_wal_have_pending());
 }
 
 /*
@@ -261,6 +335,13 @@ pgstat_create_backend(ProcNumber procnum)
 	pgstat_unlock_entry(entry_ref);
 
 	MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+
+	/*
+	 * Initialize prevBackendWalUsage with pgWalUsage so that
+	 * pgstat_backend_flush_cb() can calculate how much pgWalUsage counters
+	 * are increased by subtracting prevBackendWalUsage from pgWalUsage.
+	 */
+	prevBackendWalUsage = pgWalUsage;
 }
 
 /*