Refactor the fsync queue for wider use.

Previously, md.c and checkpointer.c were tightly integrated so that
fsync calls could be handed off and processed in the background.
Introduce a system of callbacks and file tags, so that other modules
can hand off fsync work in the same way.

For now only md.c uses the new interface, but other users are being
proposed.  Since there may be use cases that are not strictly SMGR
implementations, use a new function table for sync handlers rather
than extending the traditional SMGR one.

Instead of using a bitmapset of segment numbers for each RelFileNode
in the checkpointer's hash table, make the segment number part of the
key.  This requires sending explicit "forget" requests for every
segment individually when relations are dropped, but suits the file
layout schemes of proposed future users better (ie sparse or high
segment numbers).

Author: Shawn Debnath and Thomas Munro
Reviewed-by: Thomas Munro, Andres Freund
Discussion: https://postgr.es/m/CAEepm=2gTANm=e3ARnJT=n0h8hf88wqmaZxk0JYkxw+b21fNrw@mail.gmail.com

Author: macdice

src/backend/access/transam/twophase.c

@@ -98,6 +98,7 @@
 #include "replication/walsender.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"
+#include "storage/md.h"
 #include "storage/predicate.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"

src/backend/access/transam/xact.c

@@ -50,6 +50,7 @@
 #include "storage/fd.h"
 #include "storage/freespace.h"
 #include "storage/lmgr.h"
+#include "storage/md.h"
 #include "storage/predicate.h"
 #include "storage/proc.h"
 #include "storage/procarray.h"

src/backend/access/transam/xlog.c

@@ -66,6 +66,7 @@
 #include "storage/reinit.h"
 #include "storage/smgr.h"
 #include "storage/spin.h"
+#include "storage/sync.h"
 #include "utils/builtins.h"
 #include "utils/guc.h"
 #include "utils/memutils.h"
@@ -6981,7 +6982,7 @@ StartupXLOG(void)
 		if (ArchiveRecoveryRequested && IsUnderPostmaster)
 		{
 			PublishStartupProcessInformation();
-			SetForwardFsyncRequests();
+			EnableSyncRequestForwarding();
 			SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
 			bgwriterLaunched = true;
 		}
@@ -8566,7 +8567,7 @@ CreateCheckPoint(int flags)
 	 * the REDO pointer.  Note that smgr must not do anything that'd have to
 	 * be undone if we decide no checkpoint is needed.
 	 */
-	smgrpreckpt();
+	SyncPreCheckpoint();
 
 	/* Begin filling in the checkpoint WAL record */
 	MemSet(&checkPoint, 0, sizeof(checkPoint));
@@ -8856,7 +8857,7 @@ CreateCheckPoint(int flags)
 	/*
 	 * Let smgr do post-checkpoint cleanup (eg, deleting old files).
 	 */
-	smgrpostckpt();
+	SyncPostCheckpoint();
 
 	/*
 	 * Update the average distance between checkpoints if the prior checkpoint

src/backend/commands/dbcommands.c

@@ -54,6 +54,7 @@
 #include "storage/fd.h"
 #include "storage/lmgr.h"
 #include "storage/ipc.h"
+#include "storage/md.h"
 #include "storage/procarray.h"
 #include "storage/smgr.h"
 #include "utils/acl.h"
@@ -941,11 +942,11 @@ dropdb(const char *dbname, bool missing_ok)
 	 * worse, it will delete files that belong to a newly created database
 	 * with the same OID.
 	 */
-	ForgetDatabaseFsyncRequests(db_id);
+	ForgetDatabaseSyncRequests(db_id);
 
 	/*
 	 * Force a checkpoint to make sure the checkpointer has received the
-	 * message sent by ForgetDatabaseFsyncRequests. On Windows, this also
+	 * message sent by ForgetDatabaseSyncRequests. On Windows, this also
 	 * ensures that background procs don't hold any open files, which would
 	 * cause rmdir() to fail.
 	 */
@@ -2150,7 +2151,7 @@ dbase_redo(XLogReaderState *record)
 		DropDatabaseBuffers(xlrec->db_id);
 
 		/* Also, clean out any fsync requests that might be pending in md.c */
-		ForgetDatabaseFsyncRequests(xlrec->db_id);
+		ForgetDatabaseSyncRequests(xlrec->db_id);
 
 		/* Clean out the xlog relcache too */
 		XLogDropDatabase(xlrec->db_id);

src/backend/postmaster/checkpointer.c

@@ -108,10 +108,8 @@
  */
 typedef struct
 {
-	RelFileNode rnode;
-	ForkNumber	forknum;
-	BlockNumber segno;			/* see md.c for special values */
-	/* might add a real request-type field later; not needed yet */
+	SyncRequestType type;		/* request type */
+	FileTag		ftag;			/* file identifier */
 } CheckpointerRequest;
 
 typedef struct
@@ -349,7 +347,7 @@ CheckpointerMain(void)
 		/*
 		 * Process any requests or signals received recently.
 		 */
-		AbsorbFsyncRequests();
+		AbsorbSyncRequests();
 
 		if (got_SIGHUP)
 		{
@@ -684,7 +682,7 @@ CheckpointWriteDelay(int flags, double progress)
 			UpdateSharedMemoryConfig();
 		}
 
-		AbsorbFsyncRequests();
+		AbsorbSyncRequests();
 		absorb_counter = WRITES_PER_ABSORB;
 
 		CheckArchiveTimeout();
@@ -709,7 +707,7 @@ CheckpointWriteDelay(int flags, double progress)
 		 * operations even when we don't sleep, to prevent overflow of the
 		 * fsync request queue.
 		 */
-		AbsorbFsyncRequests();
+		AbsorbSyncRequests();
 		absorb_counter = WRITES_PER_ABSORB;
 	}
 }
@@ -1084,7 +1082,7 @@ RequestCheckpoint(int flags)
 }
 
 /*
- * ForwardFsyncRequest
+ * ForwardSyncRequest
  *		Forward a file-fsync request from a backend to the checkpointer
  *
  * Whenever a backend is compelled to write directly to a relation
@@ -1093,15 +1091,6 @@ RequestCheckpoint(int flags)
  * is dirty and must be fsync'd before next checkpoint.  We also use this
  * opportunity to count such writes for statistical purposes.
  *
- * This functionality is only supported for regular (not backend-local)
- * relations, so the rnode argument is intentionally RelFileNode not
- * RelFileNodeBackend.
- *
- * segno specifies which segment (not block!) of the relation needs to be
- * fsync'd.  (Since the valid range is much less than BlockNumber, we can
- * use high values for special flags; that's all internal to md.c, which
- * see for details.)
- *
  * To avoid holding the lock for longer than necessary, we normally write
  * to the requests[] queue without checking for duplicates.  The checkpointer
  * will have to eliminate dups internally anyway.  However, if we discover
@@ -1113,7 +1102,7 @@ RequestCheckpoint(int flags)
  * let the backend know by returning false.
  */
 bool
-ForwardFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
+ForwardSyncRequest(const FileTag *ftag, SyncRequestType type)
 {
 	CheckpointerRequest *request;
 	bool		too_full;
@@ -1122,7 +1111,7 @@ ForwardFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
 		return false;			/* probably shouldn't even get here */
 
 	if (AmCheckpointerProcess())
-		elog(ERROR, "ForwardFsyncRequest must not be called in checkpointer");
+		elog(ERROR, "ForwardSyncRequest must not be called in checkpointer");
 
 	LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);
 
@@ -1151,9 +1140,8 @@ ForwardFsyncRequest(RelFileNode rnode, ForkNumber forknum, BlockNumber segno)
 
 	/* OK, insert request */
 	request = &CheckpointerShmem->requests[CheckpointerShmem->num_requests++];
-	request->rnode = rnode;
-	request->forknum = forknum;
-	request->segno = segno;
+	request->ftag = *ftag;
+	request->type = type;
 
 	/* If queue is more than half full, nudge the checkpointer to empty it */
 	too_full = (CheckpointerShmem->num_requests >=
@@ -1284,16 +1272,16 @@ CompactCheckpointerRequestQueue(void)
 }
 
 /*
- * AbsorbFsyncRequests
- *		Retrieve queued fsync requests and pass them to local smgr.
+ * AbsorbSyncRequests
+ *		Retrieve queued sync requests and pass them to sync mechanism.
  *
  * This is exported because it must be called during CreateCheckPoint;
  * we have to be sure we have accepted all pending requests just before
  * we start fsync'ing.  Since CreateCheckPoint sometimes runs in
  * non-checkpointer processes, do nothing if not checkpointer.
  */
 void
-AbsorbFsyncRequests(void)
+AbsorbSyncRequests(void)
 {
 	CheckpointerRequest *requests = NULL;
 	CheckpointerRequest *request;
@@ -1335,7 +1323,7 @@ AbsorbFsyncRequests(void)
 	LWLockRelease(CheckpointerCommLock);
 
 	for (request = requests; n > 0; request++, n--)
-		RememberFsyncRequest(request->rnode, request->forknum, request->segno);
+		RememberSyncRequest(&request->ftag, request->type);
 
 	END_CRIT_SECTION();
 

src/backend/storage/Makefile

@@ -8,6 +8,6 @@ subdir = src/backend/storage
 top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
-SUBDIRS     = buffer file freespace ipc large_object lmgr page smgr
+SUBDIRS     = buffer file freespace ipc large_object lmgr page smgr sync
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/storage/buffer/bufmgr.c

@@ -2584,7 +2584,7 @@ CheckPointBuffers(int flags)
 	BufferSync(flags);
 	CheckpointStats.ckpt_sync_t = GetCurrentTimestamp();
 	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_SYNC_START();
-	smgrsync();
+	ProcessSyncRequests();
 	CheckpointStats.ckpt_sync_end_t = GetCurrentTimestamp();
 	TRACE_POSTGRESQL_BUFFER_CHECKPOINT_DONE();
 }