Introduce a maintenance_io_concurrency setting.

Introduce a GUC and a tablespace option to control I/O prefetching, much
like effective_io_concurrency, but for work that is done on behalf of
many client sessions.

Use the new setting in heapam.c instead of the hard-coded formula
effective_io_concurrency + 10 introduced by commit 558a9165e0.  Go with
a default value of 10 for now, because it's a round number pretty close
to the value used for that existing case.

Discussion: https://postgr.es/m/CA%2BhUKGJUw08dPs_3EUcdO6M90GnjofPYrWp4YSLaBkgYwS-AqA%40mail.gmail.com

src/backend/access/common/reloptions.c

@@ -349,6 +349,19 @@ static relopt_int intRelOpts[] =
 		-1, 0, MAX_IO_CONCURRENCY
 #else
 		0, 0, 0
+#endif
+	},
+	{
+		{
+			"maintenance_io_concurrency",
+			"Number of simultaneous requests that can be handled efficiently by the disk subsystem for maintenance work.",
+			RELOPT_KIND_TABLESPACE,
+			ShareUpdateExclusiveLock
+		},
+#ifdef USE_PREFETCH
+		-1, 0, MAX_IO_CONCURRENCY
+#else
+		0, 0, 0
 #endif
 	},
 	{
@@ -1700,7 +1713,8 @@ tablespace_reloptions(Datum reloptions, bool validate)
 	static const relopt_parse_elt tab[] = {
 		{"random_page_cost", RELOPT_TYPE_REAL, offsetof(TableSpaceOpts, random_page_cost)},
 		{"seq_page_cost", RELOPT_TYPE_REAL, offsetof(TableSpaceOpts, seq_page_cost)},
-		{"effective_io_concurrency", RELOPT_TYPE_INT, offsetof(TableSpaceOpts, effective_io_concurrency)}
+		{"effective_io_concurrency", RELOPT_TYPE_INT, offsetof(TableSpaceOpts, effective_io_concurrency)},
+		{"maintenance_io_concurrency", RELOPT_TYPE_INT, offsetof(TableSpaceOpts, maintenance_io_concurrency)}
 	};
 
 	return (bytea *) build_reloptions(reloptions, validate,

src/backend/access/heap/heapam.c

@@ -7003,7 +7003,6 @@ heap_compute_xid_horizon_for_tuples(Relation rel,
 	Page		hpage;
 #ifdef USE_PREFETCH
 	XidHorizonPrefetchState prefetch_state;
-	int			io_concurrency;
 	int			prefetch_distance;
 #endif
 
@@ -7026,24 +7025,15 @@ heap_compute_xid_horizon_for_tuples(Relation rel,
 	/*
 	 * Compute the prefetch distance that we will attempt to maintain.
 	 *
-	 * We don't use the regular formula to determine how much to prefetch
-	 * here, but instead just add a constant to effective_io_concurrency.
-	 * That's because it seems best to do some prefetching here even when
-	 * effective_io_concurrency is set to 0, but if the DBA thinks it's OK to
-	 * do more prefetching for other operations, then it's probably OK to do
-	 * more prefetching in this case, too. It may be that this formula is too
-	 * simplistic, but at the moment there is no evidence of that or any idea
-	 * about what would work better.
-	 *
 	 * Since the caller holds a buffer lock somewhere in rel, we'd better make
 	 * sure that isn't a catalog relation before we call code that does
 	 * syscache lookups, to avoid risk of deadlock.
 	 */
 	if (IsCatalogRelation(rel))
-		io_concurrency = effective_io_concurrency;
+		prefetch_distance = maintenance_io_concurrency;
 	else
-		io_concurrency = get_tablespace_io_concurrency(rel->rd_rel->reltablespace);
-	prefetch_distance = Min((io_concurrency) + 10, MAX_IO_CONCURRENCY);
+		prefetch_distance =
+			get_tablespace_maintenance_io_concurrency(rel->rd_rel->reltablespace);
 
 	/* Start prefetching. */
 	xid_horizon_prefetch_buffer(rel, &prefetch_state, prefetch_distance);

src/backend/storage/buffer/bufmgr.c

@@ -119,6 +119,13 @@ bool		track_io_timing = false;
  */
 int			effective_io_concurrency = 0;
 
+/*
+ * Like effective_io_concurrency, but used by maintenance code paths that might
+ * benefit from a higher setting because they work on behalf of many sessions.
+ * Overridden by the tablespace setting of the same name.
+ */
+int			maintenance_io_concurrency = 0;
+
 /*
  * GUC variables about triggering kernel writeback for buffers written; OS
  * dependent defaults are set via the GUC mechanism.

src/backend/utils/cache/spccache.c

@@ -221,3 +221,17 @@ get_tablespace_io_concurrency(Oid spcid)
 	else
 		return spc->opts->effective_io_concurrency;
 }
+
+/*
+ * get_tablespace_maintenance_io_concurrency
+ */
+int
+get_tablespace_maintenance_io_concurrency(Oid spcid)
+{
+	TableSpaceCacheEntry *spc = get_tablespace(spcid);
+
+	if (!spc->opts || spc->opts->maintenance_io_concurrency < 0)
+		return maintenance_io_concurrency;
+	else
+		return spc->opts->maintenance_io_concurrency;
+}

src/backend/utils/misc/guc.c

@@ -196,6 +196,7 @@ static bool check_autovacuum_max_workers(int *newval, void **extra, GucSource so
 static bool check_max_wal_senders(int *newval, void **extra, GucSource source);
 static bool check_autovacuum_work_mem(int *newval, void **extra, GucSource source);
 static bool check_effective_io_concurrency(int *newval, void **extra, GucSource source);
+static bool check_maintenance_io_concurrency(int *newval, void **extra, GucSource source);
 static void assign_pgstat_temp_directory(const char *newval, void *extra);
 static bool check_application_name(char **newval, void **extra, GucSource source);
 static void assign_application_name(const char *newval, void *extra);
@@ -2884,6 +2885,24 @@ static struct config_int ConfigureNamesInt[] =
 		check_effective_io_concurrency, NULL, NULL
 	},
 
+	{
+		{"maintenance_io_concurrency",
+			PGC_USERSET,
+			RESOURCES_ASYNCHRONOUS,
+			gettext_noop("A variant of effective_io_concurrency that is used for maintenance work."),
+			NULL,
+			GUC_EXPLAIN
+		},
+		&maintenance_io_concurrency,
+#ifdef USE_PREFETCH
+		10,
+#else
+		0,
+#endif
+		0, MAX_IO_CONCURRENCY,
+		check_maintenance_io_concurrency, NULL, NULL
+	},
+
 	{
 		{"backend_flush_after", PGC_USERSET, RESOURCES_ASYNCHRONOUS,
 			gettext_noop("Number of pages after which previously performed writes are flushed to disk."),
@@ -11466,6 +11485,19 @@ check_effective_io_concurrency(int *newval, void **extra, GucSource source)
 	return true;
 }
 
+static bool
+check_maintenance_io_concurrency(int *newval, void **extra, GucSource source)
+{
+#ifndef USE_PREFETCH
+	if (*newval != 0)
+	{
+		GUC_check_errdetail("maintenance_io_concurrency must be set to 0 on platforms that lack posix_fadvise().");
+		return false;
+	}
+#endif							/* USE_PREFETCH */
+	return true;
+}
+
 static void
 assign_pgstat_temp_directory(const char *newval, void *extra)
 {

src/bin/psql/tab-complete.c

@@ -2140,7 +2140,7 @@ psql_completion(const char *text, int start, int end)
 	/* ALTER TABLESPACE <foo> SET|RESET ( */
 	else if (Matches("ALTER", "TABLESPACE", MatchAny, "SET|RESET", "("))
 		COMPLETE_WITH("seq_page_cost", "random_page_cost",
-					  "effective_io_concurrency");
+					  "effective_io_concurrency", "maintenance_io_concurrency");
 
 	/* ALTER TEXT SEARCH */
 	else if (Matches("ALTER", "TEXT", "SEARCH"))

src/include/commands/tablespace.h

@@ -40,6 +40,7 @@ typedef struct TableSpaceOpts
 	float8		random_page_cost;
 	float8		seq_page_cost;
 	int			effective_io_concurrency;
+	int			maintenance_io_concurrency;
 } TableSpaceOpts;
 
 extern Oid	CreateTableSpace(CreateTableSpaceStmt *stmt);

src/include/storage/bufmgr.h

@@ -58,6 +58,7 @@ extern int	bgwriter_lru_maxpages;
 extern double bgwriter_lru_multiplier;
 extern bool track_io_timing;
 extern int	effective_io_concurrency;
+extern int	maintenance_io_concurrency;
 
 extern int	checkpoint_flush_after;
 extern int	backend_flush_after;

src/include/utils/spccache.h

@@ -16,5 +16,6 @@
 void		get_tablespace_page_costs(Oid spcid, float8 *spc_random_page_cost,
 									  float8 *spc_seq_page_cost);
 int			get_tablespace_io_concurrency(Oid spcid);
+int			get_tablespace_maintenance_io_concurrency(Oid spcid);
 
 #endif							/* SPCCACHE_H */