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

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 +799,18 @@ 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.  This may result in sync requests
+	 * queued for later handling by ProcessSyncRequests(), as part of the
+	 * checkpoint.
+	 */
+	SimpleLruWriteAll(CommitTsCtl, true);
 }
 
 /*
@@ -1077,3 +1066,12 @@ 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);
+}