v6-0002-Defer-flushing-of-SLRU-files.patch

From ae51604c43e10ba6d24b628c2b907eca251177e9 Mon Sep 17 00:00:00 2001
From: Thomas Munro <tmunro@postgresql.org>
Date: Mon, 21 Sep 2020 09:43:32 +1200
Subject: [PATCH v6 2/2] 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 causes a significant improvement to recovery
performance.

Remove the Shutdown{CLOG,CommitTS,SUBTRANS,MultiXact}() functions,
because they were redundant after the shutdown checkpoint that
immediately precedes them.

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.  Also rearrange things so that data collected in
CheckpointStats includes SLRU activity.

Tested-by: Jakub Wartak <Jakub.Wartak@tomtom.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com
---
 src/backend/access/transam/clog.c      |  36 +++---
 src/backend/access/transam/commit_ts.c |  32 +++--
 src/backend/access/transam/multixact.c |  53 +++++----
 src/backend/access/transam/slru.c      | 154 +++++++++++++++++--------
 src/backend/access/transam/subtrans.c  |  25 +---
 src/backend/access/transam/xlog.c      |  18 ++-
 src/backend/commands/async.c           |   5 +-
 src/backend/storage/buffer/bufmgr.c    |   7 --
 src/backend/storage/lmgr/predicate.c   |   8 +-
 src/backend/storage/sync/sync.c        |  28 ++++-
 src/include/access/clog.h              |   3 +
 src/include/access/commit_ts.h         |   3 +
 src/include/access/multixact.h         |   4 +
 src/include/access/slru.h              |  14 ++-
 src/include/storage/sync.h             |   7 +-
 src/tools/pgindent/typedefs.list       |   1 +
 16 files changed, 233 insertions(+), 165 deletions(-)

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 9e352d2658..c4c20f508b 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);
 }
 
 /*
@@ -808,34 +810,15 @@ TrimCLOG(void)
 	LWLockRelease(XactSLRULock);
 }
 
-/*
- * 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 */
 	TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
-	SimpleLruFlush(XactCtl, true);
+	SimpleLruWriteAll(XactCtl, true);
 	TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
 }
 
@@ -1026,3 +1009,12 @@ 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);
+}
diff --git a/src/backend/access/transam/commit_ts.c b/src/backend/access/transam/commit_ts.c
index f6a7329ba3..af5d9baae5 100644
--- a/src/backend/access/transam/commit_ts.c
+++ b/src/backend/access/transam/commit_ts.c
@@ -555,7 +555,8 @@ CommitTsShmemInit(void)
 	CommitTsCtl->PagePrecedes = CommitTsPagePrecedes;
 	SimpleLruInit(CommitTsCtl, "CommitTs", CommitTsShmemBuffers(), 0,
 				  CommitTsSLRULock, "pg_commit_ts",
-				  LWTRANCHE_COMMITTS_BUFFER);
+				  LWTRANCHE_COMMITTS_BUFFER,
+				  SYNC_HANDLER_COMMIT_TS);
 
 	commitTsShared = ShmemInitStruct("CommitTs shared",
 									 sizeof(CommitTimestampShared),
@@ -798,30 +799,14 @@ DeactivateCommitTs(void)
 	LWLockRelease(CommitTsSLRULock);
 }
 
-/*
- * This must be called ONCE during postmaster or standalone-backend shutdown
- */
-void
-ShutdownCommitTs(void)
-{
-	/* Flush dirty CommitTs pages to disk */
-	SimpleLruFlush(CommitTsCtl, false);
-
-	/*
-	 * fsync pg_commit_ts to ensure that any files flushed previously are
-	 * durably on disk.
-	 */
-	fsync_fname("pg_commit_ts", true);
-}
-
 /*
  * Perform a checkpoint --- either during shutdown, or on-the-fly
  */
 void
 CheckPointCommitTs(void)
 {
-	/* Flush dirty CommitTs pages to disk */
-	SimpleLruFlush(CommitTsCtl, true);
+	/* Write dirty CommitTs pages to disk */
+	SimpleLruWriteAll(CommitTsCtl, true);
 }
 
 /*
@@ -1077,3 +1062,12 @@ commit_ts_redo(XLogReaderState *record)
 	else
 		elog(PANIC, "commit_ts_redo: unknown op code %u", info);
 }
+
+/*
+ * Entrypoint for sync.c to sync commit_ts files.
+ */
+int
+committssyncfiletag(const FileTag *ftag, char *path)
+{
+	return SlruSyncFileTag(CommitTsCtl, ftag, path);
+}
diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index b8bedca04a..5501da088b 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -1831,11 +1831,13 @@ MultiXactShmemInit(void)
 	SimpleLruInit(MultiXactOffsetCtl,
 				  "MultiXactOffset", NUM_MULTIXACTOFFSET_BUFFERS, 0,
 				  MultiXactOffsetSLRULock, "pg_multixact/offsets",
-				  LWTRANCHE_MULTIXACTOFFSET_BUFFER);
+				  LWTRANCHE_MULTIXACTOFFSET_BUFFER,
+				  SYNC_HANDLER_MULTIXACT_OFFSET);
 	SimpleLruInit(MultiXactMemberCtl,
 				  "MultiXactMember", NUM_MULTIXACTMEMBER_BUFFERS, 0,
 				  MultiXactMemberSLRULock, "pg_multixact/members",
-				  LWTRANCHE_MULTIXACTMEMBER_BUFFER);
+				  LWTRANCHE_MULTIXACTMEMBER_BUFFER,
+				  SYNC_HANDLER_MULTIXACT_MEMBER);
 
 	/* Initialize our shared state struct */
 	MultiXactState = ShmemInitStruct("Shared MultiXact State",
@@ -2100,19 +2102,6 @@ TrimMultiXact(void)
 	SetMultiXactIdLimit(oldestMXact, oldestMXactDB, true);
 }
 
-/*
- * This must be called ONCE during postmaster or standalone-backend shutdown
- */
-void
-ShutdownMultiXact(void)
-{
-	/* Flush dirty MultiXact pages to disk */
-	TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(false);
-	SimpleLruFlush(MultiXactOffsetCtl, false);
-	SimpleLruFlush(MultiXactMemberCtl, false);
-	TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(false);
-}
-
 /*
  * Get the MultiXact data to save in a checkpoint record
  */
@@ -2143,9 +2132,9 @@ CheckPointMultiXact(void)
 {
 	TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_START(true);
 
-	/* Flush dirty MultiXact pages to disk */
-	SimpleLruFlush(MultiXactOffsetCtl, true);
-	SimpleLruFlush(MultiXactMemberCtl, true);
+	/* Write dirty MultiXact pages to disk */
+	SimpleLruWriteAll(MultiXactOffsetCtl, true);
+	SimpleLruWriteAll(MultiXactMemberCtl, true);
 
 	TRACE_POSTGRESQL_MULTIXACT_CHECKPOINT_DONE(true);
 }
@@ -2728,14 +2717,10 @@ find_multixact_start(MultiXactId multi, MultiXactOffset *result)
 	entryno = MultiXactIdToOffsetEntry(multi);
 
 	/*
-	 * Flush out dirty data, so PhysicalPageExists can work correctly.
-	 * SimpleLruFlush() is a pretty big hammer for that.  Alternatively we
-	 * could add an in-memory version of page exists, but find_multixact_start
-	 * is called infrequently, and it doesn't seem bad to flush buffers to
-	 * disk before truncation.
+	 * Write out dirty data, so PhysicalPageExists can work correctly.
 	 */
-	SimpleLruFlush(MultiXactOffsetCtl, true);
-	SimpleLruFlush(MultiXactMemberCtl, true);
+	SimpleLruWriteAll(MultiXactOffsetCtl, true);
+	SimpleLruWriteAll(MultiXactMemberCtl, true);
 
 	if (!SimpleLruDoesPhysicalPageExist(MultiXactOffsetCtl, pageno))
 		return false;
@@ -3386,3 +3371,21 @@ pg_get_multixact_members(PG_FUNCTION_ARGS)
 
 	SRF_RETURN_DONE(funccxt);
 }
+
+/*
+ * Entrypoint for sync.c to sync offsets files.
+ */
+int
+multixactoffsetssyncfiletag(const FileTag *ftag, char *path)
+{
+	return SlruSyncFileTag(MultiXactOffsetCtl, ftag, path);
+}
+
+/*
+ * Entrypoint for sync.c to sync members files.
+ */
+int
+multixactmemberssyncfiletag(const FileTag *ftag, char *path)
+{
+	return SlruSyncFileTag(MultiXactMemberCtl, ftag, path);
+}
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 89ce7c43b5..390d36d03f 100644
--- a/src/backend/access/transam/slru.c
+++ b/src/backend/access/transam/slru.c
@@ -63,22 +63,33 @@
 	snprintf(path, MAXPGPATH, "%s/%04X", (ctl)->Dir, seg)
 
 /*
- * During SimpleLruFlush(), we will usually not need to write/fsync more
- * than one or two physical files, but we may need to write several pages
- * per file.  We can consolidate the I/O requests by leaving files open
- * until control returns to SimpleLruFlush().  This data structure remembers
- * which files are open.
+ * During SimpleLruWriteAll(), we will usually not need to write more than one
+ * or two physical files, but we may need to write several pages per file.  We
+ * can consolidate the I/O requests by leaving files open until control returns
+ * to SimpleLruWriteAll().  This data structure remembers which files are open.
  */
-#define MAX_FLUSH_BUFFERS	16
+#define MAX_WRITEALL_BUFFERS	16
 
-typedef struct SlruFlushData
+typedef struct SlruWriteAllData
 {
 	int			num_files;		/* # files actually open */
-	int			fd[MAX_FLUSH_BUFFERS];	/* their FD's */
-	int			segno[MAX_FLUSH_BUFFERS];	/* their log seg#s */
-} SlruFlushData;
+	int			fd[MAX_WRITEALL_BUFFERS];		/* their FD's */
+	int			segno[MAX_WRITEALL_BUFFERS];	/* their log seg#s */
+} SlruWriteAllData;
 
-typedef struct SlruFlushData *SlruFlush;
+typedef struct SlruWriteAllData *SlruWriteAll;
+
+/*
+ * Populate a file tag describing a segment file.  We only use the segment
+ * number, since we can derive everything else we need by having separate
+ * sync handler functions for clog, multixact etc.
+ */
+#define INIT_SLRUFILETAG(a,xx_handler,xx_segno) \
+( \
+	memset(&(a), 0, sizeof(FileTag)), \
+	(a).handler = (xx_handler), \
+	(a).segno = (xx_segno) \
+)
 
 /*
  * Macro to mark a buffer slot "most recently used".  Note multiple evaluation
@@ -125,10 +136,10 @@ static int	slru_errno;
 
 static void SimpleLruZeroLSNs(SlruCtl ctl, int slotno);
 static void SimpleLruWaitIO(SlruCtl ctl, int slotno);
-static void SlruInternalWritePage(SlruCtl ctl, int slotno, SlruFlush fdata);
+static void SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata);
 static bool SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno);
 static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno,
-								  SlruFlush fdata);
+								  SlruWriteAll fdata);
 static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid);
 static int	SlruSelectLRUPage(SlruCtl ctl, int pageno);
 
@@ -173,7 +184,8 @@ SimpleLruShmemSize(int nslots, int nlsns)
  */
 void
 SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
-			  LWLock *ctllock, const char *subdir, int tranche_id)
+			  LWLock *ctllock, const char *subdir, int tranche_id,
+			  SyncRequestHandler sync_handler)
 {
 	SlruShared	shared;
 	bool		found;
@@ -251,7 +263,7 @@ SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
 	 * assume caller set PagePrecedes.
 	 */
 	ctl->shared = shared;
-	ctl->do_fsync = true;		/* default behavior */
+	ctl->sync_handler = sync_handler;
 	strlcpy(ctl->Dir, subdir, sizeof(ctl->Dir));
 }
 
@@ -523,7 +535,7 @@ SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
  * Control lock must be held at entry, and will be held at exit.
  */
 static void
-SlruInternalWritePage(SlruCtl ctl, int slotno, SlruFlush fdata)
+SlruInternalWritePage(SlruCtl ctl, int slotno, SlruWriteAll fdata)
 {
 	SlruShared	shared = ctl->shared;
 	int			pageno = shared->page_number[slotno];
@@ -587,6 +599,10 @@ SlruInternalWritePage(SlruCtl ctl, int slotno, SlruFlush fdata)
 	/* Now it's okay to ereport if we failed */
 	if (!ok)
 		SlruReportIOError(ctl, pageno, InvalidTransactionId);
+
+	/* If part of a checkpoint, count this as a buffer written. */
+	if (fdata)
+		CheckpointStats.ckpt_bufs_written++;
 }
 
 /*
@@ -730,13 +746,13 @@ SlruPhysicalReadPage(SlruCtl ctl, int pageno, int slotno)
  *
  * For now, assume it's not worth keeping a file pointer open across
  * independent read/write operations.  We do batch operations during
- * SimpleLruFlush, though.
+ * SimpleLruWriteAll, though.
  *
  * fdata is NULL for a standalone write, pointer to open-file info during
- * SimpleLruFlush.
+ * SimpleLruWriteAll.
  */
 static bool
-SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruFlush fdata)
+SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruWriteAll fdata)
 {
 	SlruShared	shared = ctl->shared;
 	int			segno = pageno / SLRU_PAGES_PER_SEGMENT;
@@ -791,7 +807,7 @@ SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruFlush fdata)
 	}
 
 	/*
-	 * During a Flush, we may already have the desired file open.
+	 * During a WriteAll, we may already have the desired file open.
 	 */
 	if (fdata)
 	{
@@ -837,7 +853,7 @@ SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruFlush fdata)
 
 		if (fdata)
 		{
-			if (fdata->num_files < MAX_FLUSH_BUFFERS)
+			if (fdata->num_files < MAX_WRITEALL_BUFFERS)
 			{
 				fdata->fd[fdata->num_files] = fd;
 				fdata->segno[fdata->num_files] = segno;
@@ -870,23 +886,31 @@ SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno, SlruFlush fdata)
 	}
 	pgstat_report_wait_end();
 
-	/*
-	 * If not part of Flush, need to fsync now.  We assume this happens
-	 * infrequently enough that it's not a performance issue.
-	 */
-	if (!fdata)
+	/* Queue up a sync request for the checkpointer. */
+	if (ctl->sync_handler != SYNC_HANDLER_NONE)
 	{
-		pgstat_report_wait_start(WAIT_EVENT_SLRU_SYNC);
-		if (ctl->do_fsync && pg_fsync(fd) != 0)
+		FileTag		tag;
+
+		INIT_SLRUFILETAG(tag, ctl->sync_handler, segno);
+		if (!RegisterSyncRequest(&tag, SYNC_REQUEST, false))
 		{
+			/* No space to enqueue sync request.  Do it synchronously. */
+			pgstat_report_wait_start(WAIT_EVENT_SLRU_SYNC);
+			if (pg_fsync(fd) != 0)
+			{
+				pgstat_report_wait_end();
+				slru_errcause = SLRU_FSYNC_FAILED;
+				slru_errno = errno;
+				CloseTransientFile(fd);
+				return false;
+			}
 			pgstat_report_wait_end();
-			slru_errcause = SLRU_FSYNC_FAILED;
-			slru_errno = errno;
-			CloseTransientFile(fd);
-			return false;
 		}
-		pgstat_report_wait_end();
+	}
 
+	/* Close file, unless part of flush request. */
+	if (!fdata)
+	{
 		if (CloseTransientFile(fd) != 0)
 		{
 			slru_errcause = SLRU_CLOSE_FAILED;
@@ -1122,13 +1146,16 @@ SlruSelectLRUPage(SlruCtl ctl, int pageno)
 }
 
 /*
- * Flush dirty pages to disk during checkpoint or database shutdown
+ * Write dirty pages to disk during checkpoint or database shutdown.  Flushing
+ * is deferred until the next call to ProcessSyncRequests(), though we do fsync
+ * the containing directory here to make sure that newly created directory
+ * entries are on disk.
  */
 void
-SimpleLruFlush(SlruCtl ctl, bool allow_redirtied)
+SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied)
 {
 	SlruShared	shared = ctl->shared;
-	SlruFlushData fdata;
+	SlruWriteAllData fdata;
 	int			slotno;
 	int			pageno = 0;
 	int			i;
@@ -1162,21 +1189,11 @@ SimpleLruFlush(SlruCtl ctl, bool allow_redirtied)
 	LWLockRelease(shared->ControlLock);
 
 	/*
-	 * Now fsync and close any files that were open
+	 * Now close any files that were open
 	 */
 	ok = true;
 	for (i = 0; i < fdata.num_files; i++)
 	{
-		pgstat_report_wait_start(WAIT_EVENT_SLRU_FLUSH_SYNC);
-		if (ctl->do_fsync && pg_fsync(fdata.fd[i]) != 0)
-		{
-			slru_errcause = SLRU_FSYNC_FAILED;
-			slru_errno = errno;
-			pageno = fdata.segno[i] * SLRU_PAGES_PER_SEGMENT;
-			ok = false;
-		}
-		pgstat_report_wait_end();
-
 		if (CloseTransientFile(fdata.fd[i]) != 0)
 		{
 			slru_errcause = SLRU_CLOSE_FAILED;
@@ -1192,7 +1209,7 @@ SimpleLruFlush(SlruCtl ctl, bool allow_redirtied)
 	 * Make sure that the directory entries for any newly created files are on
 	 * disk.
 	 */
-	if (ctl->do_fsync)
+	if (ctl->sync_handler != SYNC_HANDLER_NONE)
 		fsync_fname(ctl->Dir, true);
 }
 
@@ -1353,6 +1370,19 @@ restart:
 	snprintf(path, MAXPGPATH, "%s/%04X", ctl->Dir, segno);
 	ereport(DEBUG2,
 			(errmsg("removing file \"%s\"", path)));
+
+	/*
+	 * Tell the checkpointer to forget any sync requests, before we unlink the
+	 * file.
+	 */
+	if (ctl->sync_handler != SYNC_HANDLER_NONE)
+	{
+		FileTag		tag;
+
+		INIT_SLRUFILETAG(tag, ctl->sync_handler, segno);
+		RegisterSyncRequest(&tag, SYNC_FORGET_REQUEST, true);
+	}
+
 	unlink(path);
 
 	LWLockRelease(shared->ControlLock);
@@ -1451,3 +1481,31 @@ SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
 
 	return retval;
 }
+
+/*
+ * Individual SLRUs (clog, ...) have to provide a sync.c handler function so
+ * that they can provide the correct "SlruCtl" (otherwise we don't know how to
+ * build the path), but they just forward to this common implementation that
+ * performs the fsync.
+ */
+int
+SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path)
+{
+	int			fd;
+	int			save_errno;
+	int			result;
+
+	SlruFileName(ctl, path, ftag->segno);
+
+	fd = OpenTransientFile(path, O_RDWR | PG_BINARY);
+	if (fd < 0)
+		return -1;
+
+	result = pg_fsync(fd);
+	save_errno = errno;
+
+	CloseTransientFile(fd);
+
+	errno = save_errno;
+	return result;
+}
diff --git a/src/backend/access/transam/subtrans.c b/src/backend/access/transam/subtrans.c
index a50f60b99a..0111e867c7 100644
--- a/src/backend/access/transam/subtrans.c
+++ b/src/backend/access/transam/subtrans.c
@@ -193,9 +193,7 @@ SUBTRANSShmemInit(void)
 	SubTransCtl->PagePrecedes = SubTransPagePrecedes;
 	SimpleLruInit(SubTransCtl, "Subtrans", NUM_SUBTRANS_BUFFERS, 0,
 				  SubtransSLRULock, "pg_subtrans",
-				  LWTRANCHE_SUBTRANS_BUFFER);
-	/* Override default assumption that writes should be fsync'd */
-	SubTransCtl->do_fsync = false;
+				  LWTRANCHE_SUBTRANS_BUFFER, SYNC_HANDLER_NONE);
 }
 
 /*
@@ -278,23 +276,6 @@ StartupSUBTRANS(TransactionId oldestActiveXID)
 	LWLockRelease(SubtransSLRULock);
 }
 
-/*
- * This must be called ONCE during postmaster or standalone-backend shutdown
- */
-void
-ShutdownSUBTRANS(void)
-{
-	/*
-	 * Flush dirty SUBTRANS pages to disk
-	 *
-	 * This is not actually necessary from a correctness point of view. We do
-	 * it merely as a debugging aid.
-	 */
-	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_START(false);
-	SimpleLruFlush(SubTransCtl, false);
-	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_DONE(false);
-}
-
 /*
  * Perform a checkpoint --- either during shutdown, or on-the-fly
  */
@@ -302,14 +283,14 @@ void
 CheckPointSUBTRANS(void)
 {
 	/*
-	 * Flush dirty SUBTRANS pages to disk
+	 * Write dirty SUBTRANS pages to disk
 	 *
 	 * This is not actually necessary from a correctness point of view. We do
 	 * it merely to improve the odds that writing of dirty pages is done by
 	 * the checkpoint process and not by backends.
 	 */
 	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_START(true);
-	SimpleLruFlush(SubTransCtl, true);
+	SimpleLruWriteAll(SubTransCtl, true);
 	TRACE_POSTGRESQL_SUBTRANS_CHECKPOINT_DONE(true);
 }
 
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 61754312e2..ed17dd0d12 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -8528,10 +8528,6 @@ ShutdownXLOG(int code, Datum arg)
 
 		CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
 	}
-	ShutdownCLOG();
-	ShutdownCommitTs();
-	ShutdownSUBTRANS();
-	ShutdownMultiXact();
 }
 
 /*
@@ -9176,16 +9172,28 @@ CreateEndOfRecoveryRecord(void)
 static void
 CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
 {
+	/* Write out all dirty data in SLRUs and the main buffer pool */
+	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
+	CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
 	CheckPointCLOG();
 	CheckPointCommitTs();
 	CheckPointSUBTRANS();
 	CheckPointMultiXact();
 	CheckPointPredicate();
+	CheckPointBuffers(flags);
+
+	/* Perform all required fsyncs */
+	CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
+	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
+	ProcessSyncRequests();
+	CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
+	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
+
+	/* Other activities performed at checkpoint time */
 	CheckPointRelationMap();
 	CheckPointReplicationSlots();
 	CheckPointSnapBuild();
 	CheckPointLogicalRewriteHeap();
-	CheckPointBuffers(flags);	/* performs all required fsyncs */
 	CheckPointReplicationOrigin();
 	/* We deliberately delay 2PC checkpointing as long as possible */
 	CheckPointTwoPhase(checkPointRedo);
diff --git a/src/backend/commands/async.c b/src/backend/commands/async.c
index cb341365df..8dbcace3f9 100644
--- a/src/backend/commands/async.c
+++ b/src/backend/commands/async.c
@@ -554,9 +554,8 @@ AsyncShmemInit(void)
 	 */
 	NotifyCtl->PagePrecedes = asyncQueuePagePrecedes;
 	SimpleLruInit(NotifyCtl, "Notify", NUM_NOTIFY_BUFFERS, 0,
-				  NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER);
-	/* Override default assumption that writes should be fsync'd */
-	NotifyCtl->do_fsync = false;
+				  NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER,
+				  SYNC_HANDLER_NONE);
 
 	if (!found)
 	{
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index a2a963bd5b..e549fa1d30 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -2636,14 +2636,7 @@ PrintBufferLeakWarning(Buffer buffer)
 void
 CheckPointBuffers(int flags)
 {
-	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_START(flags);
-	CheckpointStats.ckpt_write_t = GetCurrentTimestamp();
 	BufferSync(flags);
-	CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
-	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
-	ProcessSyncRequests();
-	CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
-	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
 }
 
 
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
index a2f8e7524b..8a365b400c 100644
--- a/src/backend/storage/lmgr/predicate.c
+++ b/src/backend/storage/lmgr/predicate.c
@@ -821,9 +821,7 @@ SerialInit(void)
 	SerialSlruCtl->PagePrecedes = SerialPagePrecedesLogically;
 	SimpleLruInit(SerialSlruCtl, "Serial",
 				  NUM_SERIAL_BUFFERS, 0, SerialSLRULock, "pg_serial",
-				  LWTRANCHE_SERIAL_BUFFER);
-	/* Override default assumption that writes should be fsync'd */
-	SerialSlruCtl->do_fsync = false;
+				  LWTRANCHE_SERIAL_BUFFER, SYNC_HANDLER_NONE);
 
 	/*
 	 * Create or attach to the SerialControl structure.
@@ -1052,7 +1050,7 @@ CheckPointPredicate(void)
 	SimpleLruTruncate(SerialSlruCtl, tailPage);
 
 	/*
-	 * Flush dirty SLRU pages to disk
+	 * Write dirty SLRU pages to disk
 	 *
 	 * This is not actually necessary from a correctness point of view. We do
 	 * it merely as a debugging aid.
@@ -1061,7 +1059,7 @@ CheckPointPredicate(void)
 	 * before deleting the file in which they sit, which would be completely
 	 * pointless.
 	 */
-	SimpleLruFlush(SerialSlruCtl, true);
+	SimpleLruWriteAll(SerialSlruCtl, true);
 }
 
 /*------------------------------------------------------------------------*/
diff --git a/src/backend/storage/sync/sync.c b/src/backend/storage/sync/sync.c
index 3ded2cdd71..1d635d596c 100644
--- a/src/backend/storage/sync/sync.c
+++ b/src/backend/storage/sync/sync.c
@@ -18,6 +18,9 @@
 #include <fcntl.h>
 #include <sys/file.h>
 
+#include "access/commit_ts.h"
+#include "access/clog.h"
+#include "access/multixact.h"
 #include "access/xlog.h"
 #include "access/xlogutils.h"
 #include "commands/tablespace.h"
@@ -90,12 +93,31 @@ typedef struct SyncOps
 										const FileTag *candidate);
 } SyncOps;
 
+/*
+ * These indexes must correspond to the values of the SyncRequestHandler enum.
+ */
 static const SyncOps syncsw[] = {
 	/* magnetic disk */
-	{
+	[SYNC_HANDLER_MD] = {
 		.sync_syncfiletag = mdsyncfiletag,
 		.sync_unlinkfiletag = mdunlinkfiletag,
 		.sync_filetagmatches = mdfiletagmatches
+	},
+	/* pg_xact */
+	[SYNC_HANDLER_CLOG] = {
+		.sync_syncfiletag = clogsyncfiletag
+	},
+	/* pg_commit_ts */
+	[SYNC_HANDLER_COMMIT_TS] = {
+		.sync_syncfiletag = committssyncfiletag
+	},
+	/* pg_multixact/offsets */
+	[SYNC_HANDLER_MULTIXACT_OFFSET] = {
+		.sync_syncfiletag = multixactoffsetssyncfiletag
+	},
+	/* pg_multixact/members */
+	[SYNC_HANDLER_MULTIXACT_MEMBER] = {
+		.sync_syncfiletag = multixactmemberssyncfiletag
 	}
 };
 
@@ -505,8 +527,8 @@ RememberSyncRequest(const FileTag *ftag, SyncRequestType type)
 												  (void *) ftag,
 												  HASH_ENTER,
 												  &found);
-		/* if new entry, initialize it */
-		if (!found)
+		/* if new entry, or was previously canceled, initialize it */
+		if (!found || entry->canceled)
 		{
 			entry->cycle_ctr = sync_cycle_ctr;
 			entry->canceled = false;
diff --git a/src/include/access/clog.h b/src/include/access/clog.h
index 2db8acb189..d97b9042dc 100644
--- a/src/include/access/clog.h
+++ b/src/include/access/clog.h
@@ -12,6 +12,7 @@
 #define CLOG_H
 
 #include "access/xlogreader.h"
+#include "storage/sync.h"
 #include "lib/stringinfo.h"
 
 /*
@@ -50,6 +51,8 @@ extern void CheckPointCLOG(void);
 extern void ExtendCLOG(TransactionId newestXact);
 extern void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid);
 
+extern int clogsyncfiletag(const FileTag *ftag, char *path);
+
 /* XLOG stuff */
 #define CLOG_ZEROPAGE		0x00
 #define CLOG_TRUNCATE		0x10
diff --git a/src/include/access/commit_ts.h b/src/include/access/commit_ts.h
index 2740c02a84..27900ce430 100644
--- a/src/include/access/commit_ts.h
+++ b/src/include/access/commit_ts.h
@@ -14,6 +14,7 @@
 #include "access/xlog.h"
 #include "datatype/timestamp.h"
 #include "replication/origin.h"
+#include "storage/sync.h"
 #include "utils/guc.h"
 
 
@@ -45,6 +46,8 @@ extern void SetCommitTsLimit(TransactionId oldestXact,
 							 TransactionId newestXact);
 extern void AdvanceOldestCommitTsXid(TransactionId oldestXact);
 
+extern int committssyncfiletag(const FileTag *ftag, char *path);
+
 /* XLOG stuff */
 #define COMMIT_TS_ZEROPAGE		0x00
 #define COMMIT_TS_TRUNCATE		0x10
diff --git a/src/include/access/multixact.h b/src/include/access/multixact.h
index 6d729008c6..71d6e78063 100644
--- a/src/include/access/multixact.h
+++ b/src/include/access/multixact.h
@@ -13,6 +13,7 @@
 
 #include "access/xlogreader.h"
 #include "lib/stringinfo.h"
+#include "storage/sync.h"
 
 
 /*
@@ -116,6 +117,9 @@ extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
 extern bool MultiXactIdPrecedesOrEquals(MultiXactId multi1,
 										MultiXactId multi2);
 
+extern int multixactoffsetssyncfiletag(const FileTag *ftag, char *path);
+extern int multixactmemberssyncfiletag(const FileTag *ftag, char *path);
+
 extern void AtEOXact_MultiXact(void);
 extern void AtPrepare_MultiXact(void);
 extern void PostPrepare_MultiXact(TransactionId xid);
diff --git a/src/include/access/slru.h b/src/include/access/slru.h
index 61fbc80ef0..b39b43504d 100644
--- a/src/include/access/slru.h
+++ b/src/include/access/slru.h
@@ -15,6 +15,7 @@
 
 #include "access/xlogdefs.h"
 #include "storage/lwlock.h"
+#include "storage/sync.h"
 
 
 /*
@@ -111,10 +112,10 @@ typedef struct SlruCtlData
 	SlruShared	shared;
 
 	/*
-	 * This flag tells whether to fsync writes (true for pg_xact and multixact
-	 * stuff, false for pg_subtrans and pg_notify).
+	 * Which sync handler function to use when handing sync requests over to
+	 * the checkpointer.  SYNC_HANDLER_NONE to disable fsync (eg pg_notify).
 	 */
-	bool		do_fsync;
+	SyncRequestHandler sync_handler;
 
 	/*
 	 * Decide which of two page numbers is "older" for truncation purposes. We
@@ -135,14 +136,15 @@ typedef SlruCtlData *SlruCtl;
 
 extern Size SimpleLruShmemSize(int nslots, int nlsns);
 extern void SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
-						  LWLock *ctllock, const char *subdir, int tranche_id);
+						  LWLock *ctllock, const char *subdir, int tranche_id,
+						  SyncRequestHandler sync_handler);
 extern int	SimpleLruZeroPage(SlruCtl ctl, int pageno);
 extern int	SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok,
 							  TransactionId xid);
 extern int	SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno,
 									   TransactionId xid);
 extern void SimpleLruWritePage(SlruCtl ctl, int slotno);
-extern void SimpleLruFlush(SlruCtl ctl, bool allow_redirtied);
+extern void SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied);
 extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);
 extern bool SimpleLruDoesPhysicalPageExist(SlruCtl ctl, int pageno);
 
@@ -151,6 +153,8 @@ typedef bool (*SlruScanCallback) (SlruCtl ctl, char *filename, int segpage,
 extern bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data);
 extern void SlruDeleteSegment(SlruCtl ctl, int segno);
 
+extern int	SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path);
+
 /* SlruScanDirectory public callbacks */
 extern bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename,
 										int segpage, void *data);
diff --git a/src/include/storage/sync.h b/src/include/storage/sync.h
index e16ab8e711..f32e412e75 100644
--- a/src/include/storage/sync.h
+++ b/src/include/storage/sync.h
@@ -34,7 +34,12 @@ typedef enum SyncRequestType
  */
 typedef enum SyncRequestHandler
 {
-	SYNC_HANDLER_MD = 0			/* md smgr */
+	SYNC_HANDLER_MD = 0,
+	SYNC_HANDLER_CLOG,
+	SYNC_HANDLER_COMMIT_TS,
+	SYNC_HANDLER_MULTIXACT_OFFSET,
+	SYNC_HANDLER_MULTIXACT_MEMBER,
+	SYNC_HANDLER_NONE
 } SyncRequestHandler;
 
 /*
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index b1afb345c3..88fc3ceae4 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -2404,6 +2404,7 @@ Syn
 SyncOps
 SyncRepConfigData
 SyncRepStandbyData
+SyncRequestHandler
 SyncRequestType
 SysScanDesc
 SyscacheCallbackFunction
-- 
2.20.1