Defer flushing of SLRU files.

Previously, we called fsync() after writing out individual pg_xact,
pg_multixact and pg_commit_ts pages due to cache pressure, leading to
regular I/O stalls in user backends and recovery.  Collapse requests for
the same file into a single system call as part of the next checkpoint,
as we already did for relation files, using the infrastructure developed
by commit 3eb77eba.  This can cause a significant improvement to
recovery performance, especially when it's otherwise CPU-bound.

Hoist ProcessSyncRequests() up into CheckPointGuts() to make it clearer
that it applies to all the SLRU mini-buffer-pools as well as the main
buffer pool.  Rearrange things so that data collected in CheckpointStats
includes SLRU activity.

Also remove the Shutdown{CLOG,CommitTS,SUBTRANS,MultiXact}() functions,
because they were redundant after the shutdown checkpoint that
immediately precedes them.  (I'm not sure if they were ever needed, but
they aren't now.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> (parts)
Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com

1 files changed, 18 insertions, 22 deletions

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 9e352d26583..034349aa7b9 100644
--- a/src/backend/access/transam/clog.c
+++ b/src/backend/access/transam/clog.c
@@ -42,6 +42,7 @@
 #include "pg_trace.h"
 #include "pgstat.h"
 #include "storage/proc.h"
+#include "storage/sync.h"
 
 /*
  * Defines for CLOG page sizes.  A page is the same BLCKSZ as is used
@@ -691,7 +692,8 @@ CLOGShmemInit(void)
 {
 	XactCtl->PagePrecedes = CLOGPagePrecedes;
 	SimpleLruInit(XactCtl, "Xact", CLOGShmemBuffers(), CLOG_LSNS_PER_PAGE,
-				  XactSLRULock, "pg_xact", LWTRANCHE_XACT_BUFFER);
+				  XactSLRULock, "pg_xact", LWTRANCHE_XACT_BUFFER,
+				  SYNC_HANDLER_CLOG);
 }
 
 /*
@@ -809,33 +811,18 @@ TrimCLOG(void)
 }
 
 /*
- * This must be called ONCE during postmaster or standalone-backend shutdown
- */
-void
-ShutdownCLOG(void)
-{
-	/* Flush dirty CLOG pages to disk */
-	TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(false);
-	SimpleLruFlush(XactCtl, false);
-
-	/*
-	 * fsync pg_xact to ensure that any files flushed previously are durably
-	 * on disk.
-	 */
-	fsync_fname("pg_xact", true);
-
-	TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(false);
-}
-
-/*
  * Perform a checkpoint --- either during shutdown, or on-the-fly
  */
 void
 CheckPointCLOG(void)
 {
-	/* Flush dirty CLOG pages to disk */
+	/*
+	 * Write dirty CLOG pages to disk.  This may result in sync requests
+	 * queued for later handling by ProcessSyncRequests(), as part of the
+	 * checkpoint.
+	 */
 	TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
-	SimpleLruFlush(XactCtl, true);
+	SimpleLruWriteAll(XactCtl, true);
 	TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
 }
 
@@ -1026,3 +1013,12 @@ clog_redo(XLogReaderState *record)
 	else
 		elog(PANIC, "clog_redo: unknown op code %u", info);
 }
+
+/*
+ * Entrypoint for sync.c to sync clog files.
+ */
+int
+clogsyncfiletag(const FileTag *ftag, char *path)
+{
+	return SlruSyncFileTag(XactCtl, ftag, path);
+}