Re: [HACKERS] WAL logging problem in 9.4.3? - Mailing list pgsql-hackers

Hello.  I found that c203d6cf81 hit this and this is the rebased
version on the current master.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center
From 3dac5baf787dc949cfb22a698a0d72b6eb48e75e Mon Sep 17 00:00:00 2001
From: Kyotaro Horiguchi <horiguchi.kyotaro@lab.ntt.co.jp>
Date: Tue, 12 Sep 2017 13:01:33 +0900
Subject: [PATCH] Fix WAL logging problem

---
 src/backend/access/heap/heapam.c        | 113 ++++++++---
 src/backend/access/heap/pruneheap.c     |   3 +-
 src/backend/access/heap/rewriteheap.c   |   4 +-
 src/backend/access/heap/visibilitymap.c |   3 +-
 src/backend/access/transam/xact.c       |   7 +
 src/backend/catalog/storage.c           | 320 +++++++++++++++++++++++++++++---
 src/backend/commands/copy.c             |  13 +-
 src/backend/commands/createas.c         |   9 +-
 src/backend/commands/matview.c          |   6 +-
 src/backend/commands/tablecmds.c        |   8 +-
 src/backend/commands/vacuumlazy.c       |   6 +-
 src/backend/storage/buffer/bufmgr.c     |  40 +++-
 src/backend/utils/cache/relcache.c      |  13 ++
 src/include/access/heapam.h             |   8 +-
 src/include/catalog/storage.h           |   5 +-
 src/include/storage/bufmgr.h            |   2 +
 src/include/utils/rel.h                 |   8 +
 17 files changed, 480 insertions(+), 88 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index d7279248e7..8fd2c2948e 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -34,6 +34,28 @@
  *      the POSTGRES heap access method used for all POSTGRES
  *      relations.
  *
+ * WAL CONSIDERATIONS
+ *      All heap operations are normally WAL-logged. but there are a few
+ *      exceptions. Temporary and unlogged relations never need to be
+ *      WAL-logged, but we can also skip WAL-logging for a table that was
+ *      created in the same transaction, if we don't need WAL for PITR or
+ *      WAL archival purposes (i.e. if wal_level=minimal), and we fsync()
+ *      the file to disk at COMMIT instead.
+ *
+ *      The same-relation optimization is not employed automatically on all
+ *      updates to a table that was created in the same transacton, because
+ *      for a small number of changes, it's cheaper to just create the WAL
+ *      records than fsyncing() the whole relation at COMMIT. It is only
+ *      worthwhile for (presumably) large operations like COPY, CLUSTER,
+ *      or VACUUM FULL. Use heap_register_sync() to initiate such an
+ *      operation; it will cause any subsequent updates to the table to skip
+ *      WAL-logging, if possible, and cause the heap to be synced to disk at
+ *      COMMIT.
+ *
+ *      To make that work, all modifications to heap must use
+ *      HeapNeedsWAL() to check if WAL-logging is needed in this transaction
+ *      for the given block.
+ *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
@@ -57,6 +79,7 @@
 #include "catalog/catalog.h"
 #include "catalog/namespace.h"
 #include "catalog/index.h"
+#include "catalog/storage.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "port/atomics.h"
@@ -2400,12 +2423,6 @@ ReleaseBulkInsertStatePin(BulkInsertState bistate)
  * The new tuple is stamped with current transaction ID and the specified
  * command ID.
  *
- * If the HEAP_INSERT_SKIP_WAL option is specified, the new tuple is not
- * logged in WAL, even for a non-temp relation.  Safe usage of this behavior
- * requires that we arrange that all new tuples go into new pages not
- * containing any tuples from other transactions, and that the relation gets
- * fsync'd before commit.  (See also heap_sync() comments)
- *
  * The HEAP_INSERT_SKIP_FSM option is passed directly to
  * RelationGetBufferForTuple, which see for more info.
  *
@@ -2436,6 +2453,7 @@ ReleaseBulkInsertStatePin(BulkInsertState bistate)
  * TID where the tuple was stored.  But note that any toasting of fields
  * within the tuple data is NOT reflected into *tup.
  */
+extern HTAB *pendingSyncs;
 Oid
 heap_insert(Relation relation, HeapTuple tup, CommandId cid,
             int options, BulkInsertState bistate)
@@ -2509,7 +2527,7 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid,
     MarkBufferDirty(buffer);
 
     /* XLOG stuff */
-    if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, buffer))
     {
         xl_heap_insert xlrec;
         xl_heap_header xlhdr;
@@ -2710,12 +2728,10 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
     int            ndone;
     char       *scratch = NULL;
     Page        page;
-    bool        needwal;
     Size        saveFreeSpace;
     bool        need_tuple_data = RelationIsLogicallyLogged(relation);
     bool        need_cids = RelationIsAccessibleInLogicalDecoding(relation);
 
-    needwal = !(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation);
     saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
                                                    HEAP_DEFAULT_FILLFACTOR);
 
@@ -2730,7 +2746,7 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
      * palloc() within a critical section is not safe, so we allocate this
      * beforehand.
      */
-    if (needwal)
+    if (RelationNeedsWAL(relation))
         scratch = palloc(BLCKSZ);
 
     /*
@@ -2765,6 +2781,7 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
         Buffer        vmbuffer = InvalidBuffer;
         bool        all_visible_cleared = false;
         int            nthispage;
+        bool        needwal;
 
         CHECK_FOR_INTERRUPTS();
 
@@ -2776,6 +2793,7 @@ heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
                                            InvalidBuffer, options, bistate,
                                            &vmbuffer, NULL);
         page = BufferGetPage(buffer);
+        needwal = BufferNeedsWAL(relation, buffer);
 
         /* NO EREPORT(ERROR) from here till changes are logged */
         START_CRIT_SECTION();
@@ -3332,7 +3350,7 @@ l1:
      * NB: heap_abort_speculative() uses the same xlog record and replay
      * routines.
      */
-    if (RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, buffer))
     {
         xl_heap_delete xlrec;
         XLogRecPtr    recptr;
@@ -4307,7 +4325,8 @@ l2:
     MarkBufferDirty(buffer);
 
     /* XLOG stuff */
-    if (RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, buffer) ||
+        BufferNeedsWAL(relation, newbuf))
     {
         XLogRecPtr    recptr;
 
@@ -5276,7 +5295,7 @@ failed:
      * (Also, in a PITR log-shipping or 2PC environment, we have to have XLOG
      * entries for everything anyway.)
      */
-    if (RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, *buffer))
     {
         xl_heap_lock xlrec;
         XLogRecPtr    recptr;
@@ -6020,7 +6039,7 @@ l4:
         MarkBufferDirty(buf);
 
         /* XLOG stuff */
-        if (RelationNeedsWAL(rel))
+        if (BufferNeedsWAL(rel, buf))
         {
             xl_heap_lock_updated xlrec;
             XLogRecPtr    recptr;
@@ -6174,7 +6193,7 @@ heap_finish_speculative(Relation relation, HeapTuple tuple)
     htup->t_ctid = tuple->t_self;
 
     /* XLOG stuff */
-    if (RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, buffer))
     {
         xl_heap_confirm xlrec;
         XLogRecPtr    recptr;
@@ -6307,7 +6326,7 @@ heap_abort_speculative(Relation relation, HeapTuple tuple)
      * The WAL records generated here match heap_delete().  The same recovery
      * routines are used.
      */
-    if (RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, buffer))
     {
         xl_heap_delete xlrec;
         XLogRecPtr    recptr;
@@ -6416,7 +6435,7 @@ heap_inplace_update(Relation relation, HeapTuple tuple)
     MarkBufferDirty(buffer);
 
     /* XLOG stuff */
-    if (RelationNeedsWAL(relation))
+    if (BufferNeedsWAL(relation, buffer))
     {
         xl_heap_inplace xlrec;
         XLogRecPtr    recptr;
@@ -7602,7 +7621,7 @@ log_heap_clean(Relation reln, Buffer buffer,
     XLogRecPtr    recptr;
 
     /* Caller should not call me on a non-WAL-logged relation */
-    Assert(RelationNeedsWAL(reln));
+    Assert(BufferNeedsWAL(reln, buffer));
 
     xlrec.latestRemovedXid = latestRemovedXid;
     xlrec.nredirected = nredirected;
@@ -7650,7 +7669,7 @@ log_heap_freeze(Relation reln, Buffer buffer, TransactionId cutoff_xid,
     XLogRecPtr    recptr;
 
     /* Caller should not call me on a non-WAL-logged relation */
-    Assert(RelationNeedsWAL(reln));
+    Assert(BufferNeedsWAL(reln, buffer));
     /* nor when there are no tuples to freeze */
     Assert(ntuples > 0);
 
@@ -7735,7 +7754,7 @@ log_heap_update(Relation reln, Buffer oldbuf,
     int            bufflags;
 
     /* Caller should not call me on a non-WAL-logged relation */
-    Assert(RelationNeedsWAL(reln));
+    Assert(BufferNeedsWAL(reln, newbuf) || BufferNeedsWAL(reln, oldbuf));
 
     XLogBeginInsert();
 
@@ -8826,8 +8845,13 @@ heap_xlog_update(XLogReaderState *record, bool hot_update)
      */
 
     /* Deal with old tuple version */
-    oldaction = XLogReadBufferForRedo(record, (oldblk == newblk) ? 0 : 1,
-                                      &obuffer);
+    if (oldblk == newblk)
+        oldaction = XLogReadBufferForRedo(record, 0, &obuffer);
+    else if (XLogRecHasBlockRef(record, 1))
+        oldaction = XLogReadBufferForRedo(record, 1, &obuffer);
+    else
+        oldaction = BLK_DONE;
+
     if (oldaction == BLK_NEEDS_REDO)
     {
         page = BufferGetPage(obuffer);
@@ -8881,6 +8905,8 @@ heap_xlog_update(XLogReaderState *record, bool hot_update)
         PageInit(page, BufferGetPageSize(nbuffer), 0);
         newaction = BLK_NEEDS_REDO;
     }
+    else if (!XLogRecHasBlockRef(record, 0))
+        newaction = BLK_DONE;
     else
         newaction = XLogReadBufferForRedo(record, 0, &nbuffer);
 
@@ -9317,9 +9343,16 @@ heap2_redo(XLogReaderState *record)
  *    heap_sync        - sync a heap, for use when no WAL has been written
  *
  * This forces the heap contents (including TOAST heap if any) down to disk.
- * If we skipped using WAL, and WAL is otherwise needed, we must force the
- * relation down to disk before it's safe to commit the transaction.  This
- * requires writing out any dirty buffers and then doing a forced fsync.
+ * If we did any changes to the heap bypassing the buffer manager, we must
+ * force the relation down to disk before it's safe to commit the
+ * transaction, because the direct modifications will not be flushed by
+ * the next checkpoint.
+ *
+ * We used to also use this after batch operations like COPY and CLUSTER,
+ * if we skipped using WAL and WAL is otherwise needed, but there were
+ * corner-cases involving other WAL-logged operations to the same
+ * relation, where that was not enough. heap_register_sync() should be
+ * used for that purpose instead.
  *
  * Indexes are not touched.  (Currently, index operations associated with
  * the commands that use this are WAL-logged and so do not need fsync.
@@ -9429,3 +9462,33 @@ heap_mask(char *pagedata, BlockNumber blkno)
         }
     }
 }
+
+/*
+ *    heap_register_sync    - register a heap to be synced to disk at commit
+ *
+ * This can be used to skip WAL-logging changes on a relation file that has
+ * been created in the same transaction. This makes note of the current size of
+ * the relation, and ensures that when the relation is extended, any changes
+ * to the new blocks in the heap, in the same transaction, will not be
+ * WAL-logged. Instead, the heap contents are flushed to disk at commit,
+ * like heap_sync() does.
+ *
+ * This does the same for the TOAST heap, if any. Indexes are not affected.
+ */
+void
+heap_register_sync(Relation rel)
+{
+    /* non-WAL-logged tables never need fsync */
+    if (!RelationNeedsWAL(rel))
+        return;
+
+    RecordPendingSync(rel);
+    if (OidIsValid(rel->rd_rel->reltoastrelid))
+    {
+        Relation    toastrel;
+
+        toastrel = heap_open(rel->rd_rel->reltoastrelid, AccessShareLock);
+        RecordPendingSync(toastrel);
+        heap_close(toastrel, AccessShareLock);
+    }
+}
diff --git a/src/backend/access/heap/pruneheap.c b/src/backend/access/heap/pruneheap.c
index f67d7d15df..6dd2ae5254 100644
--- a/src/backend/access/heap/pruneheap.c
+++ b/src/backend/access/heap/pruneheap.c
@@ -20,6 +20,7 @@
 #include "access/htup_details.h"
 #include "access/xlog.h"
 #include "catalog/catalog.h"
+#include "catalog/storage.h"
 #include "miscadmin.h"
 #include "pgstat.h"
 #include "storage/bufmgr.h"
@@ -259,7 +260,7 @@ heap_page_prune(Relation relation, Buffer buffer, TransactionId OldestXmin,
         /*
          * Emit a WAL HEAP_CLEAN record showing what we did
          */
-        if (RelationNeedsWAL(relation))
+        if (BufferNeedsWAL(relation, buffer))
         {
             XLogRecPtr    recptr;
 
diff --git a/src/backend/access/heap/rewriteheap.c b/src/backend/access/heap/rewriteheap.c
index 7d466c2588..7471d7461b 100644
--- a/src/backend/access/heap/rewriteheap.c
+++ b/src/backend/access/heap/rewriteheap.c
@@ -652,9 +652,7 @@ raw_heap_insert(RewriteState state, HeapTuple tup)
     }
     else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
         heaptup = toast_insert_or_update(state->rs_new_rel, tup, NULL,
-                                         HEAP_INSERT_SKIP_FSM |
-                                         (state->rs_use_wal ?
-                                          0 : HEAP_INSERT_SKIP_WAL));
+                                         HEAP_INSERT_SKIP_FSM);
     else
         heaptup = tup;
 
diff --git a/src/backend/access/heap/visibilitymap.c b/src/backend/access/heap/visibilitymap.c
index b251e69703..4a46444f33 100644
--- a/src/backend/access/heap/visibilitymap.c
+++ b/src/backend/access/heap/visibilitymap.c
@@ -88,6 +88,7 @@
 #include "access/heapam_xlog.h"
 #include "access/visibilitymap.h"
 #include "access/xlog.h"
+#include "catalog/storage.h"
 #include "miscadmin.h"
 #include "storage/bufmgr.h"
 #include "storage/lmgr.h"
@@ -307,7 +308,7 @@ visibilitymap_set(Relation rel, BlockNumber heapBlk, Buffer heapBuf,
         map[mapByte] |= (flags << mapOffset);
         MarkBufferDirty(vmBuf);
 
-        if (RelationNeedsWAL(rel))
+        if (BufferNeedsWAL(rel, heapBuf))
         {
             if (XLogRecPtrIsInvalid(recptr))
             {
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index b88d4ccf74..976fbeb02f 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -2001,6 +2001,9 @@ CommitTransaction(void)
     /* close large objects before lower-level cleanup */
     AtEOXact_LargeObject(true);
 
+    /* Flush updates to relations that we didn't WAL-logged */
+    smgrDoPendingSyncs(true);
+
     /*
      * Mark serializable transaction as complete for predicate locking
      * purposes.  This should be done as late as we can put it and still allow
@@ -2229,6 +2232,9 @@ PrepareTransaction(void)
     /* close large objects before lower-level cleanup */
     AtEOXact_LargeObject(true);
 
+    /* Flush updates to relations that we didn't WAL-logged */
+    smgrDoPendingSyncs(true);
+
     /*
      * Mark serializable transaction as complete for predicate locking
      * purposes.  This should be done as late as we can put it and still allow
@@ -2542,6 +2548,7 @@ AbortTransaction(void)
     AtAbort_Notify();
     AtEOXact_RelationMap(false);
     AtAbort_Twophase();
+    smgrDoPendingSyncs(false);    /* abandone pending syncs */
 
     /*
      * Advertise the fact that we aborted in pg_xact (assuming that we got as
diff --git a/src/backend/catalog/storage.c b/src/backend/catalog/storage.c
index cff49bae9e..e9abd49070 100644
--- a/src/backend/catalog/storage.c
+++ b/src/backend/catalog/storage.c
@@ -29,6 +29,7 @@
 #include "catalog/storage_xlog.h"
 #include "storage/freespace.h"
 #include "storage/smgr.h"
+#include "utils/hsearch.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
 
@@ -63,6 +64,49 @@ typedef struct PendingRelDelete
 
 static PendingRelDelete *pendingDeletes = NULL; /* head of linked list */
 
+/*
+ * We also track relation files (RelFileNode values) that have been created
+ * in the same transaction, and that have been modified without WAL-logging
+ * the action (an optimization possible with wal_level=minimal). When we are
+ * about to skip WAL-logging, a PendingRelSync entry is created, and
+ * 'sync_above' is set to the current size of the relation. Any operations
+ * on blocks < sync_above need to be WAL-logged as usual, but for operations
+ * on higher blocks, WAL-logging is skipped.
+ *
+ * NB: after WAL-logging has been skipped for a block, we must not WAL-log
+ * any subsequent actions on the same block either. Replaying the WAL record
+ * of the subsequent action might fail otherwise, as the "before" state of
+ * the block might not match, as the earlier actions were not WAL-logged.
+ * Likewise, after we have WAL-logged an operation for a block, we must
+ * WAL-log any subsequent operations on the same page as well. Replaying
+ * a possible full-page-image from the earlier WAL record would otherwise
+ * revert the page to the old state, even if we sync the relation at end
+ * of transaction.
+ *
+ * If a relation is truncated (without creating a new relfilenode), and we
+ * emit a WAL record of the truncation, we can't skip WAL-logging for any
+ * of the truncated blocks anymore, as replaying the truncation record will
+ * destroy all the data inserted after that. But if we have already decided
+ * to skip WAL-logging changes to a relation, and the relation is truncated,
+ * we don't need to WAL-log the truncation either.
+ *
+ * This mechanism is currently only used by heaps. Indexes are always
+ * WAL-logged. Also, this only applies for wal_level=minimal; with higher
+ * WAL levels we need the WAL for PITR/replication anyway.
+ */
+typedef struct PendingRelSync
+{
+    RelFileNode relnode;        /* relation created in same xact */
+    BlockNumber sync_above;        /* WAL-logging skipped for blocks >=
+                                 * sync_above */
+    BlockNumber truncated_to;    /* truncation WAL record was written */
+}    PendingRelSync;
+
+/* Relations that need to be fsync'd at commit */
+static HTAB *pendingSyncs = NULL;
+
+static void createPendingSyncsHash(void);
+
 /*
  * RelationCreateStorage
  *        Create physical storage for a relation.
@@ -226,6 +270,8 @@ RelationPreserveStorage(RelFileNode rnode, bool atCommit)
 void
 RelationTruncate(Relation rel, BlockNumber nblocks)
 {
+    PendingRelSync *pending = NULL;
+    bool        found;
     bool        fsm;
     bool        vm;
 
@@ -260,37 +306,81 @@ RelationTruncate(Relation rel, BlockNumber nblocks)
      */
     if (RelationNeedsWAL(rel))
     {
-        /*
-         * Make an XLOG entry reporting the file truncation.
-         */
-        XLogRecPtr    lsn;
-        xl_smgr_truncate xlrec;
+        /* no_pending_sync is ignored since new entry is created here */
+        if (!rel->pending_sync)
+        {
+            if (!pendingSyncs)
+                createPendingSyncsHash();
+            elog(DEBUG2, "RelationTruncate: accessing hash");
+            pending = (PendingRelSync *) hash_search(pendingSyncs,
+                                                 (void *) &rel->rd_node,
+                                                 HASH_ENTER, &found);
+            if (!found)
+            {
+                pending->sync_above = InvalidBlockNumber;
+                pending->truncated_to = InvalidBlockNumber;
+            }
 
-        xlrec.blkno = nblocks;
-        xlrec.rnode = rel->rd_node;
-        xlrec.flags = SMGR_TRUNCATE_ALL;
+            rel->no_pending_sync= false;
+            rel->pending_sync = pending;
+        }
 
-        XLogBeginInsert();
-        XLogRegisterData((char *) &xlrec, sizeof(xlrec));
+        if (rel->pending_sync->sync_above == InvalidBlockNumber ||
+            rel->pending_sync->sync_above < nblocks)
+        {
+            /*
+             * Make an XLOG entry reporting the file truncation.
+             */
+            XLogRecPtr        lsn;
+            xl_smgr_truncate xlrec;
 
-        lsn = XLogInsert(RM_SMGR_ID,
-                         XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
+            xlrec.blkno = nblocks;
+            xlrec.rnode = rel->rd_node;
 
-        /*
-         * Flush, because otherwise the truncation of the main relation might
-         * hit the disk before the WAL record, and the truncation of the FSM
-         * or visibility map. If we crashed during that window, we'd be left
-         * with a truncated heap, but the FSM or visibility map would still
-         * contain entries for the non-existent heap pages.
-         */
-        if (fsm || vm)
-            XLogFlush(lsn);
+            XLogBeginInsert();
+            XLogRegisterData((char *) &xlrec, sizeof(xlrec));
+
+            lsn = XLogInsert(RM_SMGR_ID,
+                             XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
+
+            elog(DEBUG2, "WAL-logged truncation of rel %u/%u/%u to %u blocks",
+                 rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+                 nblocks);
+
+            /*
+             * Flush, because otherwise the truncation of the main relation
+             * might hit the disk before the WAL record, and the truncation of
+             * the FSM or visibility map. If we crashed during that window,
+             * we'd be left with a truncated heap, but the FSM or visibility
+             * map would still contain entries for the non-existent heap
+             * pages.
+             */
+            if (fsm || vm)
+                XLogFlush(lsn);
+
+            rel->pending_sync->truncated_to = nblocks;
+        }
     }
 
     /* Do the real work */
     smgrtruncate(rel->rd_smgr, MAIN_FORKNUM, nblocks);
 }
 
+/* create the hash table to track pending at-commit fsyncs */
+static void
+createPendingSyncsHash(void)
+{
+    /* First time through: initialize the hash table */
+    HASHCTL        ctl;
+
+    MemSet(&ctl, 0, sizeof(ctl));
+    ctl.keysize = sizeof(RelFileNode);
+    ctl.entrysize = sizeof(PendingRelSync);
+    ctl.hash = tag_hash;
+    pendingSyncs = hash_create("pending relation sync table", 5,
+                               &ctl, HASH_ELEM | HASH_FUNCTION);
+}
+
 /*
  *    smgrDoPendingDeletes() -- Take care of relation deletes at end of xact.
  *
@@ -368,6 +458,24 @@ smgrDoPendingDeletes(bool isCommit)
     }
 }
 
+/*
+ * RelationRemovePendingSync() -- remove pendingSync entry for a relation
+ */
+void
+RelationRemovePendingSync(Relation rel)
+{
+    bool found;
+
+    rel->pending_sync = NULL;
+    rel->no_pending_sync = true;
+    if (pendingSyncs)
+    {
+        elog(DEBUG2, "RelationRemovePendingSync: accessing hash");
+        hash_search(pendingSyncs, (void *) &rel->rd_node, HASH_REMOVE, &found);
+    }
+}
+
+
 /*
  * smgrGetPendingDeletes() -- Get a list of non-temp relations to be deleted.
  *
@@ -419,6 +527,176 @@ smgrGetPendingDeletes(bool forCommit, RelFileNode **ptr)
     return nrels;
 }
 
+
+/*
+ * Remember that the given relation needs to be sync'd at commit, because we
+ * are going to skip WAL-logging subsequent actions to it.
+ */
+void
+RecordPendingSync(Relation rel)
+{
+    bool found = true;
+    BlockNumber nblocks;
+
+    Assert(RelationNeedsWAL(rel));
+
+    /* ignore no_pending_sync since new entry is created here */
+    if (!rel->pending_sync)
+    {
+        if (!pendingSyncs)
+            createPendingSyncsHash();
+
+        /* Look up or create an entry */
+        rel->no_pending_sync = false;
+        elog(DEBUG2, "RecordPendingSync: accessing hash");
+        rel->pending_sync =
+            (PendingRelSync *) hash_search(pendingSyncs,
+                                           (void *) &rel->rd_node,
+                                           HASH_ENTER, &found);
+    }
+
+    nblocks = RelationGetNumberOfBlocks(rel);
+    if (!found)
+    {
+        rel->pending_sync->truncated_to = InvalidBlockNumber;
+        rel->pending_sync->sync_above = nblocks;
+
+        elog(DEBUG2,
+             "registering new pending sync for rel %u/%u/%u at block %u",
+             rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+             nblocks);
+
+    }
+    else if (rel->pending_sync->sync_above == InvalidBlockNumber)
+    {
+        elog(DEBUG2, "registering pending sync for rel %u/%u/%u at block %u",
+             rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+             nblocks);
+        rel->pending_sync->sync_above = nblocks;
+    }
+    else
+        elog(DEBUG2,
+             "pending sync for rel %u/%u/%u was already registered at block %u (new %u)",
+             rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+             rel->pending_sync->sync_above, nblocks);
+}
+
+/*
+ * Do changes to given heap page need to be WAL-logged?
+ *
+ * This takes into account any previous RecordPendingSync() requests.
+ *
+ * Note that it is required to check this before creating any WAL records for
+ * heap pages - it is not merely an optimization! WAL-logging a record, when
+ * we have already skipped a previous WAL record for the same page could lead
+ * to failure at WAL replay, as the "before" state expected by the record
+ * might not match what's on disk. Also, if the heap was truncated earlier, we
+ * must WAL-log any changes to the once-truncated blocks, because replaying
+ * the truncation record will destroy them.
+ */
+bool
+BufferNeedsWAL(Relation rel, Buffer buf)
+{
+    BlockNumber blkno = InvalidBlockNumber;
+
+    if (!RelationNeedsWAL(rel))
+        return false;
+
+    /*
+     * no point in doing further work if we know that we don't have pending
+     * sync
+     */
+    if (!pendingSyncs || rel->no_pending_sync)
+        return true;
+
+    Assert(BufferIsValid(buf));
+
+    elog(DEBUG2, "BufferNeedsWAL(r %d, b %d): hash = %p, ent=%p, neg = %d", rel->rd_id, BufferGetBlockNumber(buf),
pendingSyncs,rel->pending_sync, rel->no_pending_sync);
 
+
+    /* do the real work */
+    if (!rel->pending_sync)
+    {
+        bool found = false;
+
+        /*
+         * Hold the entry in rel. This relies on the fact that hash entry
+         * never moves.
+         */
+        rel->pending_sync =
+            (PendingRelSync *) hash_search(pendingSyncs,
+                                           (void *) &rel->rd_node,
+                                           HASH_FIND, &found);
+        elog(DEBUG2, "BufferNeedsWAL: accessing hash : %s", found ? "found" : "not found");
+        if (!found)
+        {
+            /* we don't have no one. don't access the hash no longer */
+            rel->no_pending_sync = true;
+            return true;
+        }
+    }
+
+    blkno = BufferGetBlockNumber(buf);
+    if (rel->pending_sync->sync_above == InvalidBlockNumber ||
+        rel->pending_sync->sync_above > blkno)
+    {
+        elog(DEBUG2, "not skipping WAL-logging for rel %u/%u/%u block %u, because sync_above is %u",
+             rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+             blkno, rel->pending_sync->sync_above);
+        return true;
+    }
+
+    /*
+     * We have emitted a truncation record for this block.
+     */
+    if (rel->pending_sync->truncated_to != InvalidBlockNumber &&
+        rel->pending_sync->truncated_to <= blkno)
+    {
+        elog(DEBUG2, "not skipping WAL-logging for rel %u/%u/%u block %u, because it was truncated earlier in the same
xact",
+             rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+             blkno);
+        return true;
+    }
+
+    elog(DEBUG2, "skipping WAL-logging for rel %u/%u/%u block %u",
+         rel->rd_node.spcNode, rel->rd_node.dbNode, rel->rd_node.relNode,
+         blkno);
+
+    return false;
+}
+
+/*
+ * Sync to disk any relations that we skipped WAL-logging for earlier.
+ */
+void
+smgrDoPendingSyncs(bool isCommit)
+{
+    if (!pendingSyncs)
+        return;
+
+    if (isCommit)
+    {
+        HASH_SEQ_STATUS status;
+        PendingRelSync *pending;
+
+        hash_seq_init(&status, pendingSyncs);
+
+        while ((pending = hash_seq_search(&status)) != NULL)
+        {
+            if (pending->sync_above != InvalidBlockNumber)
+            {
+                FlushRelationBuffersWithoutRelCache(pending->relnode, false);
+                smgrimmedsync(smgropen(pending->relnode, InvalidBackendId), MAIN_FORKNUM);
+
+                elog(DEBUG2, "syncing rel %u/%u/%u", pending->relnode.spcNode,
+                     pending->relnode.dbNode, pending->relnode.relNode);
+            }
+        }
+    }
+
+    hash_destroy(pendingSyncs);
+    pendingSyncs = NULL;
+}
+
 /*
  *    PostPrepare_smgr -- Clean up after a successful PREPARE
  *
diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index a42861da0d..de9fc12615 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -2352,8 +2352,7 @@ CopyFrom(CopyState cstate)
      *    - data is being written to relfilenode created in this transaction
      * then we can skip writing WAL.  It's safe because if the transaction
      * doesn't commit, we'll discard the table (or the new relfilenode file).
-     * If it does commit, we'll have done the heap_sync at the bottom of this
-     * routine first.
+     * If it does commit, commit will do heap_sync().
      *
      * As mentioned in comments in utils/rel.h, the in-same-transaction test
      * is not always set correctly, since in rare cases rd_newRelfilenodeSubid
@@ -2385,7 +2384,7 @@ CopyFrom(CopyState cstate)
     {
         hi_options |= HEAP_INSERT_SKIP_FSM;
         if (!XLogIsNeeded())
-            hi_options |= HEAP_INSERT_SKIP_WAL;
+            heap_register_sync(cstate->rel);
     }
 
     /*
@@ -2821,11 +2820,11 @@ CopyFrom(CopyState cstate)
     FreeExecutorState(estate);
 
     /*
-     * If we skipped writing WAL, then we need to sync the heap (but not
-     * indexes since those use WAL anyway)
+     * If we skipped writing WAL, then we will sync the heap at the end of
+     * the transaction. (We used to do it here, but it was later found out
+     * that to be safe, we must also avoid WAL-logging any subsequent
+     * actions on the pages we skipped WAL for). Indexes always use WAL.
      */
-    if (hi_options & HEAP_INSERT_SKIP_WAL)
-        heap_sync(cstate->rel);
 
     return processed;
 }
diff --git a/src/backend/commands/createas.c b/src/backend/commands/createas.c
index 3d82edbf58..a3c3518c69 100644
--- a/src/backend/commands/createas.c
+++ b/src/backend/commands/createas.c
@@ -567,8 +567,9 @@ intorel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
      * We can skip WAL-logging the insertions, unless PITR or streaming
      * replication is in use. We can skip the FSM in any case.
      */
-    myState->hi_options = HEAP_INSERT_SKIP_FSM |
-        (XLogIsNeeded() ? 0 : HEAP_INSERT_SKIP_WAL);
+    if (!XLogIsNeeded())
+        heap_register_sync(intoRelationDesc);
+    myState->hi_options = HEAP_INSERT_SKIP_FSM;
     myState->bistate = GetBulkInsertState();
 
     /* Not using WAL requires smgr_targblock be initially invalid */
@@ -617,9 +618,7 @@ intorel_shutdown(DestReceiver *self)
 
     FreeBulkInsertState(myState->bistate);
 
-    /* If we skipped using WAL, must heap_sync before commit */
-    if (myState->hi_options & HEAP_INSERT_SKIP_WAL)
-        heap_sync(myState->rel);
+    /* If we skipped using WAL, we will sync the relation at commit */
 
     /* close rel, but keep lock until commit */
     heap_close(myState->rel, NoLock);
diff --git a/src/backend/commands/matview.c b/src/backend/commands/matview.c
index 23892b1b81..f1b48583ba 100644
--- a/src/backend/commands/matview.c
+++ b/src/backend/commands/matview.c
@@ -464,7 +464,7 @@ transientrel_startup(DestReceiver *self, int operation, TupleDesc typeinfo)
      */
     myState->hi_options = HEAP_INSERT_SKIP_FSM | HEAP_INSERT_FROZEN;
     if (!XLogIsNeeded())
-        myState->hi_options |= HEAP_INSERT_SKIP_WAL;
+        heap_register_sync(transientrel);
     myState->bistate = GetBulkInsertState();
 
     /* Not using WAL requires smgr_targblock be initially invalid */
@@ -507,9 +507,7 @@ transientrel_shutdown(DestReceiver *self)
 
     FreeBulkInsertState(myState->bistate);
 
-    /* If we skipped using WAL, must heap_sync before commit */
-    if (myState->hi_options & HEAP_INSERT_SKIP_WAL)
-        heap_sync(myState->transientrel);
+    /* If we skipped using WAL, we will sync the relation at commit */
 
     /* close transientrel, but keep lock until commit */
     heap_close(myState->transientrel, NoLock);
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 83a881eff3..ee8c80f34f 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -4481,8 +4481,9 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
         bistate = GetBulkInsertState();
 
         hi_options = HEAP_INSERT_SKIP_FSM;
+
         if (!XLogIsNeeded())
-            hi_options |= HEAP_INSERT_SKIP_WAL;
+            heap_register_sync(newrel);
     }
     else
     {
@@ -4755,8 +4756,6 @@ ATRewriteTable(AlteredTableInfo *tab, Oid OIDNewHeap, LOCKMODE lockmode)
         FreeBulkInsertState(bistate);
 
         /* If we skipped writing WAL, then we need to sync the heap. */
-        if (hi_options & HEAP_INSERT_SKIP_WAL)
-            heap_sync(newrel);
 
         heap_close(newrel, NoLock);
     }
@@ -10811,11 +10810,12 @@ ATExecSetTableSpace(Oid tableOid, Oid newTableSpace, LOCKMODE lockmode)
 
     /*
      * Create and copy all forks of the relation, and schedule unlinking of
-     * old physical files.
+     * old physical files. Pending syncs for the old node is no longer needed.
      *
      * NOTE: any conflict in relfilenode value will be caught in
      * RelationCreateStorage().
      */
+    RelationRemovePendingSync(rel);
     RelationCreateStorage(newrnode, rel->rd_rel->relpersistence);
 
     /* copy main fork */
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index f9da24c491..78909bc519 100644
--- a/src/backend/commands/vacuumlazy.c
+++ b/src/backend/commands/vacuumlazy.c
@@ -900,7 +900,7 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
                  * page has been previously WAL-logged, and if not, do that
                  * now.
                  */
-                if (RelationNeedsWAL(onerel) &&
+                if (BufferNeedsWAL(onerel, buf) &&
                     PageGetLSN(page) == InvalidXLogRecPtr)
                     log_newpage_buffer(buf, true);
 
@@ -1170,7 +1170,7 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
             }
 
             /* Now WAL-log freezing if necessary */
-            if (RelationNeedsWAL(onerel))
+            if (BufferNeedsWAL(onerel, buf))
             {
                 XLogRecPtr    recptr;
 
@@ -1531,7 +1531,7 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, Buffer buffer,
     MarkBufferDirty(buffer);
 
     /* XLOG stuff */
-    if (RelationNeedsWAL(onerel))
+    if (BufferNeedsWAL(onerel, buffer))
     {
         XLogRecPtr    recptr;
 
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 01eabe5706..1095f6c721 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -451,6 +451,7 @@ static BufferDesc *BufferAlloc(SMgrRelation smgr,
             BufferAccessStrategy strategy,
             bool *foundPtr);
 static void FlushBuffer(BufferDesc *buf, SMgrRelation reln);
+static void FlushRelationBuffers_common(SMgrRelation smgr, bool islocal);
 static void AtProcExit_Buffers(int code, Datum arg);
 static void CheckForBufferLeaks(void);
 static int    rnode_comparator(const void *p1, const void *p2);
@@ -3147,20 +3148,41 @@ PrintPinnedBufs(void)
 void
 FlushRelationBuffers(Relation rel)
 {
-    int            i;
-    BufferDesc *bufHdr;
-
     /* Open rel at the smgr level if not already done */
     RelationOpenSmgr(rel);
 
-    if (RelationUsesLocalBuffers(rel))
+    FlushRelationBuffers_common(rel->rd_smgr, RelationUsesLocalBuffers(rel));
+}
+
+/*
+ * Like FlushRelationBuffers(), but the relation is specified by a
+ * RelFileNode
+ */
+void
+FlushRelationBuffersWithoutRelCache(RelFileNode rnode, bool islocal)
+{
+    FlushRelationBuffers_common(smgropen(rnode, InvalidBackendId), islocal);
+}
+
+/*
+ * Code shared between functions FlushRelationBuffers() and
+ * FlushRelationBuffersWithoutRelCache().
+ */
+static void
+FlushRelationBuffers_common(SMgrRelation smgr, bool islocal)
+{
+    RelFileNode rnode = smgr->smgr_rnode.node;
+    int            i;
+    BufferDesc *bufHdr;
+
+    if (islocal)
     {
         for (i = 0; i < NLocBuffer; i++)
         {
             uint32        buf_state;
 
             bufHdr = GetLocalBufferDescriptor(i);
-            if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
+            if (RelFileNodeEquals(bufHdr->tag.rnode, rnode) &&
                 ((buf_state = pg_atomic_read_u32(&bufHdr->state)) &
                  (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
             {
@@ -3177,7 +3199,7 @@ FlushRelationBuffers(Relation rel)
 
                 PageSetChecksumInplace(localpage, bufHdr->tag.blockNum);
 
-                smgrwrite(rel->rd_smgr,
+                smgrwrite(smgr,
                           bufHdr->tag.forkNum,
                           bufHdr->tag.blockNum,
                           localpage,
@@ -3207,18 +3229,18 @@ FlushRelationBuffers(Relation rel)
          * As in DropRelFileNodeBuffers, an unlocked precheck should be safe
          * and saves some cycles.
          */
-        if (!RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node))
+        if (!RelFileNodeEquals(bufHdr->tag.rnode, rnode))
             continue;
 
         ReservePrivateRefCountEntry();
 
         buf_state = LockBufHdr(bufHdr);
-        if (RelFileNodeEquals(bufHdr->tag.rnode, rel->rd_node) &&
+        if (RelFileNodeEquals(bufHdr->tag.rnode, rnode) &&
             (buf_state & (BM_VALID | BM_DIRTY)) == (BM_VALID | BM_DIRTY))
         {
             PinBuffer_Locked(bufHdr);
             LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
-            FlushBuffer(bufHdr, rel->rd_smgr);
+            FlushBuffer(bufHdr, smgr);
             LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
             UnpinBuffer(bufHdr, true);
         }
diff --git a/src/backend/utils/cache/relcache.c b/src/backend/utils/cache/relcache.c
index 48f92dc430..399390e6c1 100644
--- a/src/backend/utils/cache/relcache.c
+++ b/src/backend/utils/cache/relcache.c
@@ -76,6 +76,7 @@
 #include "pgstat.h"
 #include "rewrite/rewriteDefine.h"
 #include "rewrite/rowsecurity.h"
+#include "storage/bufmgr.h"
 #include "storage/lmgr.h"
 #include "storage/smgr.h"
 #include "utils/array.h"
@@ -417,6 +418,10 @@ AllocateRelationDesc(Form_pg_class relp)
     /* which we mark as a reference-counted tupdesc */
     relation->rd_att->tdrefcount = 1;
 
+    /* We don't know if pending sync for this relation exists so far */
+    relation->pending_sync = NULL;
+    relation->no_pending_sync = false;
+
     MemoryContextSwitchTo(oldcxt);
 
     return relation;
@@ -2072,6 +2077,10 @@ formrdesc(const char *relationName, Oid relationReltype,
         relation->rd_rel->relhasindex = true;
     }
 
+    /* We don't know if pending sync for this relation exists so far */
+    relation->pending_sync = NULL;
+    relation->no_pending_sync = false;
+
     /*
      * add new reldesc to relcache
      */
@@ -3402,6 +3411,10 @@ RelationBuildLocalRelation(const char *relname,
     else
         rel->rd_rel->relfilenode = relfilenode;
 
+    /* newly built relation has no pending sync */
+    rel->no_pending_sync = true;
+    rel->pending_sync = NULL;
+
     RelationInitLockInfo(rel);    /* see lmgr.c */
 
     RelationInitPhysicalAddr(rel);
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 4c0256b18a..fff3fd42aa 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -25,10 +25,9 @@
 
 
 /* "options" flag bits for heap_insert */
-#define HEAP_INSERT_SKIP_WAL    0x0001
-#define HEAP_INSERT_SKIP_FSM    0x0002
-#define HEAP_INSERT_FROZEN        0x0004
-#define HEAP_INSERT_SPECULATIVE 0x0008
+#define HEAP_INSERT_SKIP_FSM    0x0001
+#define HEAP_INSERT_FROZEN        0x0002
+#define HEAP_INSERT_SPECULATIVE 0x0004
 
 typedef struct BulkInsertStateData *BulkInsertState;
 
@@ -180,6 +179,7 @@ extern void simple_heap_delete(Relation relation, ItemPointer tid);
 extern void simple_heap_update(Relation relation, ItemPointer otid,
                    HeapTuple tup);
 
+extern void heap_register_sync(Relation relation);
 extern void heap_sync(Relation relation);
 extern void heap_update_snapshot(HeapScanDesc scan, Snapshot snapshot);
 
diff --git a/src/include/catalog/storage.h b/src/include/catalog/storage.h
index ef52d85803..49d93cd01f 100644
--- a/src/include/catalog/storage.h
+++ b/src/include/catalog/storage.h
@@ -22,13 +22,16 @@ extern void RelationCreateStorage(RelFileNode rnode, char relpersistence);
 extern void RelationDropStorage(Relation rel);
 extern void RelationPreserveStorage(RelFileNode rnode, bool atCommit);
 extern void RelationTruncate(Relation rel, BlockNumber nblocks);
-
+extern void RelationRemovePendingSync(Relation rel);
 /*
  * These functions used to be in storage/smgr/smgr.c, which explains the
  * naming
  */
 extern void smgrDoPendingDeletes(bool isCommit);
 extern int    smgrGetPendingDeletes(bool forCommit, RelFileNode **ptr);
+extern void smgrDoPendingSyncs(bool isCommit);
+extern void RecordPendingSync(Relation rel);
+bool BufferNeedsWAL(Relation rel, Buffer buf);
 extern void AtSubCommit_smgr(void);
 extern void AtSubAbort_smgr(void);
 extern void PostPrepare_smgr(void);
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 3cce3906a0..9fae7c6ae5 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -190,6 +190,8 @@ extern BlockNumber RelationGetNumberOfBlocksInFork(Relation relation,
                                 ForkNumber forkNum);
 extern void FlushOneBuffer(Buffer buffer);
 extern void FlushRelationBuffers(Relation rel);
+extern void FlushRelationBuffersWithoutRelCache(RelFileNode rnode,
+                                    bool islocal);
 extern void FlushDatabaseBuffers(Oid dbid);
 extern void DropRelFileNodeBuffers(RelFileNodeBackend rnode,
                        ForkNumber forkNum, BlockNumber firstDelBlock);
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index c26c395b0b..040ae3a07a 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -218,6 +218,14 @@ typedef struct RelationData
 
     /* use "struct" here to avoid needing to include pgstat.h: */
     struct PgStat_TableStatus *pgstat_info; /* statistics collection area */
+
+    /*
+     * no_pending_sync is true if this relation is known not to have pending
+     * syncs.  Elsewise searching for registered sync is required if
+     * pending_sync is NULL.
+     */
+    bool                   no_pending_sync;
+    struct PendingRelSync *pending_sync;
 } RelationData;
 
 
-- 
2.16.3