Move pgstat.tmp into a temporary directory under $PGDATA named pg_stat_tmp.

This allows the use of a ramdrive (either through mount or symlink) for
the temporary file that's written every half second, which should
reduce I/O.

On server shutdown/startup, the file is written to the old location in
the global directory, to preserve data across restarts.

Bump catversion since the $PGDATA directory layout changed.

doc/src/sgml/monitoring.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/monitoring.sgml,v 1.60 2008/06/18 17:44:12 neilc Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/monitoring.sgml,v 1.61 2008/08/05 12:09:30 mha Exp $ -->
 
 <chapter id="monitoring">
  <title>Monitoring Database Activity</title>
@@ -164,6 +164,17 @@ postgres: <replaceable>user</> <replaceable>database</> <replaceable>host</> <re
    only superusers are allowed to change these parameters with
    <command>SET</>.)
   </para>
+
+  <para>
+   The statistics collector communicates with the backends needing 
+   information (including autovacuum) through temporary files.
+   These files are stored in the <filename>pg_stat_tmp</filename> subdirectory.
+   When the postmaster shuts down, a permanent copy of the statistics
+   data is stored in the <filename>global</filename> subdirectory. For increased
+   performance, it is possible to mount or symlink a RAM based
+   filesystem to the <filename>pg_stat_tmp</filename> directory.
+  </para>
+
  </sect2>
 
  <sect2 id="monitoring-stats-views">

doc/src/sgml/storage.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.23 2008/05/02 01:08:26 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.24 2008/08/05 12:09:30 mha Exp $ -->
 
 <chapter id="storage">
 
@@ -77,6 +77,12 @@ Item
   (used for shared row locks)</entry> 
 </row>
 
+<row>
+ <entry><filename>pg_stat_tmp</></entry>
+ <entry>Subdirectory containing temporary files for the statistics
+  subsystem</entry>
+</row>
+
 <row>
  <entry><filename>pg_subtrans</></entry>
  <entry>Subdirectory containing subtransaction status data</entry>

src/backend/postmaster/pgstat.c

@@ -13,7 +13,7 @@
  *
  *	Copyright (c) 2001-2008, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.177 2008/08/01 13:16:08 alvherre Exp $
+ *	$PostgreSQL: pgsql/src/backend/postmaster/pgstat.c,v 1.178 2008/08/05 12:09:30 mha Exp $
  * ----------
  */
 #include "postgres.h"
@@ -68,8 +68,10 @@
  * Paths for the statistics files (relative to installation's $PGDATA).
  * ----------
  */
-#define PGSTAT_STAT_FILENAME	"global/pgstat.stat"
-#define PGSTAT_STAT_TMPFILE		"global/pgstat.tmp"
+#define PGSTAT_STAT_PERMANENT_FILENAME		"global/pgstat.stat"
+#define PGSTAT_STAT_PERMANENT_TMPFILE		"global/pgstat.tmp"
+#define PGSTAT_STAT_FILENAME				"pg_stat_tmp/pgstat.stat"
+#define PGSTAT_STAT_TMPFILE					"pg_stat_tmp/pgstat.tmp"
 
 /* ----------
  * Timer definitions.
@@ -219,8 +221,8 @@ static void force_statwrite(SIGNAL_ARGS);
 static void pgstat_beshutdown_hook(int code, Datum arg);
 
 static PgStat_StatDBEntry *pgstat_get_db_entry(Oid databaseid, bool create);
-static void pgstat_write_statsfile(void);
-static HTAB *pgstat_read_statsfile(Oid onlydb);
+static void pgstat_write_statsfile(bool permanent);
+static HTAB *pgstat_read_statsfile(Oid onlydb, bool permanent);
 static void backend_read_statsfile(void);
 static void pgstat_read_current_status(void);
 
@@ -510,6 +512,7 @@ void
 pgstat_reset_all(void)
 {
 	unlink(PGSTAT_STAT_FILENAME);
+	unlink(PGSTAT_STAT_PERMANENT_FILENAME);
 }
 
 #ifdef EXEC_BACKEND
@@ -2598,7 +2601,7 @@ PgstatCollectorMain(int argc, char *argv[])
 	 * zero.
 	 */
 	pgStatRunningInCollector = true;
-	pgStatDBHash = pgstat_read_statsfile(InvalidOid);
+	pgStatDBHash = pgstat_read_statsfile(InvalidOid, true);
 
 	/*
 	 * Setup the descriptor set for select(2).	Since only one bit in the set
@@ -2638,7 +2641,7 @@ PgstatCollectorMain(int argc, char *argv[])
 			if (!PostmasterIsAlive(true))
 				break;
 
-			pgstat_write_statsfile();
+			pgstat_write_statsfile(false);
 			need_statwrite = false;
 			need_timer = true;
 		}
@@ -2806,7 +2809,7 @@ PgstatCollectorMain(int argc, char *argv[])
 	/*
 	 * Save the final stats to reuse at next startup.
 	 */
-	pgstat_write_statsfile();
+	pgstat_write_statsfile(true);
 
 	exit(0);
 }
@@ -2891,10 +2894,14 @@ pgstat_get_db_entry(Oid databaseid, bool create)
  * pgstat_write_statsfile() -
  *
  *	Tell the news.
+ *	If writing to the permanent file (happens when the collector is
+ *	shutting down only), remove the temporary file so that backends
+ *	starting up under a new postmaster can't read the old data before
+ *	the new collector is ready.
  * ----------
  */
 static void
-pgstat_write_statsfile(void)
+pgstat_write_statsfile(bool permanent)
 {
 	HASH_SEQ_STATUS hstat;
 	HASH_SEQ_STATUS tstat;
@@ -2904,17 +2911,19 @@ pgstat_write_statsfile(void)
 	PgStat_StatFuncEntry *funcentry;
 	FILE	   *fpout;
 	int32		format_id;
+	const char *tmpfile = permanent?PGSTAT_STAT_PERMANENT_TMPFILE:PGSTAT_STAT_TMPFILE;
+	const char *statfile = permanent?PGSTAT_STAT_PERMANENT_FILENAME:PGSTAT_STAT_FILENAME;
 
 	/*
 	 * Open the statistics temp file to write out the current values.
 	 */
-	fpout = fopen(PGSTAT_STAT_TMPFILE, PG_BINARY_W);
+	fpout = fopen(tmpfile, PG_BINARY_W);
 	if (fpout == NULL)
 	{
 		ereport(LOG,
 				(errcode_for_file_access(),
 				 errmsg("could not open temporary statistics file \"%s\": %m",
-						PGSTAT_STAT_TMPFILE)));
+						tmpfile)));
 		return;
 	}
 
@@ -2981,26 +2990,29 @@ pgstat_write_statsfile(void)
 		ereport(LOG,
 				(errcode_for_file_access(),
 			   errmsg("could not write temporary statistics file \"%s\": %m",
-					  PGSTAT_STAT_TMPFILE)));
+					  tmpfile)));
 		fclose(fpout);
-		unlink(PGSTAT_STAT_TMPFILE);
+		unlink(tmpfile);
 	}
 	else if (fclose(fpout) < 0)
 	{
 		ereport(LOG,
 				(errcode_for_file_access(),
 			   errmsg("could not close temporary statistics file \"%s\": %m",
-					  PGSTAT_STAT_TMPFILE)));
-		unlink(PGSTAT_STAT_TMPFILE);
+					  tmpfile)));
+		unlink(tmpfile);
 	}
-	else if (rename(PGSTAT_STAT_TMPFILE, PGSTAT_STAT_FILENAME) < 0)
+	else if (rename(tmpfile, statfile) < 0)
 	{
 		ereport(LOG,
 				(errcode_for_file_access(),
 				 errmsg("could not rename temporary statistics file \"%s\" to \"%s\": %m",
-						PGSTAT_STAT_TMPFILE, PGSTAT_STAT_FILENAME)));
-		unlink(PGSTAT_STAT_TMPFILE);
+						tmpfile, statfile)));
+		unlink(tmpfile);
 	}
+
+	if (permanent)
+		unlink(PGSTAT_STAT_FILENAME);
 }
 
 
@@ -3012,7 +3024,7 @@ pgstat_write_statsfile(void)
  * ----------
  */
 static HTAB *
-pgstat_read_statsfile(Oid onlydb)
+pgstat_read_statsfile(Oid onlydb, bool permanent)
 {
 	PgStat_StatDBEntry *dbentry;
 	PgStat_StatDBEntry dbbuf;
@@ -3027,6 +3039,7 @@ pgstat_read_statsfile(Oid onlydb)
 	FILE	   *fpin;
 	int32		format_id;
 	bool		found;
+	const char *statfile = permanent?PGSTAT_STAT_PERMANENT_FILENAME:PGSTAT_STAT_FILENAME;
 
 	/*
 	 * The tables will live in pgStatLocalContext.
@@ -3055,7 +3068,7 @@ pgstat_read_statsfile(Oid onlydb)
 	 * return zero for anything and the collector simply starts from scratch
 	 * with empty counters.
 	 */
-	if ((fpin = AllocateFile(PGSTAT_STAT_FILENAME, PG_BINARY_R)) == NULL)
+	if ((fpin = AllocateFile(statfile, PG_BINARY_R)) == NULL)
 		return dbhash;
 
 	/*
@@ -3244,6 +3257,9 @@ pgstat_read_statsfile(Oid onlydb)
 done:
 	FreeFile(fpin);
 
+	if (permanent)
+		unlink(PGSTAT_STAT_PERMANENT_FILENAME);
+
 	return dbhash;
 }
 
@@ -3262,9 +3278,9 @@ backend_read_statsfile(void)
 
 	/* Autovacuum launcher wants stats about all databases */
 	if (IsAutoVacuumLauncherProcess())
-		pgStatDBHash = pgstat_read_statsfile(InvalidOid);
+		pgStatDBHash = pgstat_read_statsfile(InvalidOid, false);
 	else
-		pgStatDBHash = pgstat_read_statsfile(MyDatabaseId);
+		pgStatDBHash = pgstat_read_statsfile(MyDatabaseId, false);
 }
 
 

src/bin/initdb/initdb.c

@@ -42,7 +42,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  * Portions taken from FreeBSD.
  *
- * $PostgreSQL: pgsql/src/bin/initdb/initdb.c,v 1.158 2008/07/19 04:01:29 tgl Exp $
+ * $PostgreSQL: pgsql/src/bin/initdb/initdb.c,v 1.159 2008/08/05 12:09:30 mha Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2461,7 +2461,8 @@ main(int argc, char *argv[])
 		"pg_multixact/offsets",
 		"base",
 		"base/1",
-		"pg_tblspc"
+		"pg_tblspc",
+		"pg_stat_tmp"
 	};
 
 	progname = get_progname(argv[0]);

src/include/catalog/catversion.h

@@ -37,7 +37,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.474 2008/08/02 21:32:00 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.475 2008/08/05 12:09:30 mha Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	200808011
+#define CATALOG_VERSION_NO	200808051
 
 #endif