Move page initialization from RelationAddExtraBlocks() to use.

anarazel · anarazel · commit e6799d5a5301 · 2019-01-28T13:15:11.000-08:00
Previously we initialized pages when bulk extending in RelationAddExtraBlocks(). That has a major disadvantage: It ties RelationAddExtraBlocks() to heap, as other types of storage are likely to need different amounts of special space, have different amount of free space (previously determined by PageGetHeapFreeSpace()). That we're relying on initializing pages, but not WAL logging the initialization, also means the risk for getting "WARNING: relation \"%s\" page %u is uninitialized --- fixing" style warnings in vacuums after crashes/immediate shutdowns, is considerably higher. The warning sounds much more serious than what they are. Fix those two issues together by not initializing pages in RelationAddExtraPages() (but continue to do so in RelationGetBufferForTuple(), which is linked much more closely to heap), and accepting uninitialized pages as normal in vacuumlazy.c. When vacuumlazy encounters an empty page it now adds it to the FSM, but does nothing else. We chose to not issue a debug message, much less a warning in that case - it seems rarely useful, and quite likely to scare people unnecessarily. For now empty pages aren't added to the VM, because standbys would not re-discover such pages after a promotion. In contrast to other sources for empty pages, there's no corresponding WAL records triggering FSM updates during replay. Author: Andres Freund Reviewed-By: Tom Lane Discussion: https://postgr.es/m/20181219083945.6khtgm36mivonhva@alap3.anarazel.de
diff --git a/src/backend/access/heap/hio.c b/src/backend/access/heap/hio.c
@@ -204,7 +204,8 @@ RelationAddExtraBlocks(Relation relation, BulkInsertState bistate)
 		/*
 		 * Extend by one page.  This should generally match the main-line
 		 * extension code in RelationGetBufferForTuple, except that we hold
-		 * the relation extension lock throughout.
+		 * the relation extension lock throughout, and we don't immediately
+		 * initialize the page (see below).
 		 */
 		buffer = ReadBufferBI(relation, P_NEW, bistate);
 
@@ -216,18 +217,16 @@ RelationAddExtraBlocks(Relation relation, BulkInsertState bistate)
 				 BufferGetBlockNumber(buffer),
 				 RelationGetRelationName(relation));
 
-		PageInit(page, BufferGetPageSize(buffer), 0);
-
 		/*
-		 * We mark all the new buffers dirty, but do nothing to write them
-		 * out; they'll probably get used soon, and even if they are not, a
-		 * crash will leave an okay all-zeroes page on disk.
+		 * Add the page to the FSM without initializing. If we were to
+		 * initialize here the page would potentially get flushed out to disk
+		 * before we add any useful content. There's no guarantee that that'd
+		 * happen before a potential crash, so we need to deal with
+		 * uninitialized pages anyway, thus avoid the potential for
+		 * unnecessary writes.
 		 */
-		MarkBufferDirty(buffer);
-
-		/* we'll need this info below */
 		blockNum = BufferGetBlockNumber(buffer);
-		freespace = PageGetHeapFreeSpace(page);
+		freespace = BufferGetPageSize(buffer) - SizeOfPageHeaderData;
 
 		UnlockReleaseBuffer(buffer);
 
@@ -479,6 +478,18 @@ RelationGetBufferForTuple(Relation relation, Size len,
 		 * we're done.
 		 */
 		page = BufferGetPage(buffer);
+
+		/*
+		 * Initialize page, it'll be used soon.  We could avoid dirtying the
+		 * buffer here, and rely on the caller to do so whenever it puts a
+		 * tuple onto the page, but there seems not much benefit in doing so.
+		 */
+		if (PageIsNew(page))
+		{
+			PageInit(page, BufferGetPageSize(buffer), 0);
+			MarkBufferDirty(buffer);
+		}
+
 		pageFreeSpace = PageGetHeapFreeSpace(page);
 		if (len + saveFreeSpace <= pageFreeSpace)
 		{
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
@@ -861,42 +861,38 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
 		if (PageIsNew(page))
 		{
 			/*
-			 * An all-zeroes page could be left over if a backend extends the
-			 * relation but crashes before initializing the page. Reclaim such
-			 * pages for use.
+			 * All-zeroes pages can be left over if either a backend extends
+			 * the relation by a single page, but crashes before the newly
+			 * initialized page has been written out, or when bulk-extending
+			 * the relation (which creates a number of empty pages at the tail
+			 * end of the relation, but enters them into the FSM).
 			 *
-			 * We have to be careful here because we could be looking at a
-			 * page that someone has just added to the relation and not yet
-			 * been able to initialize (see RelationGetBufferForTuple). To
-			 * protect against that, release the buffer lock, grab the
-			 * relation extension lock momentarily, and re-lock the buffer. If
-			 * the page is still uninitialized by then, it must be left over
-			 * from a crashed backend, and we can initialize it.
+			 * Make sure these pages are in the FSM, to ensure they can be
+			 * reused. Do that by testing if there's any space recorded for
+			 * the page. If not, enter it.
 			 *
-			 * We don't really need the relation lock when this is a new or
-			 * temp relation, but it's probably not worth the code space to
-			 * check that, since this surely isn't a critical path.
-			 *
-			 * Note: the comparable code in vacuum.c need not worry because
-			 * it's got exclusive lock on the whole relation.
+			 * Note we do not enter the page into the visibilitymap. That has
+			 * the downside that we repeatedly visit this page in subsequent
+			 * vacuums, but otherwise we'll never not discover the space on a
+			 * promoted standby. The harm of repeated checking ought to
+			 * normally not be too bad - the space usually should be used at
+			 * some point, otherwise there wouldn't be any regular vacuums.
+			 */
+			empty_pages++;
+
+			/*
+			 * Perform checking of FSM after releasing lock, the fsm is
+			 * approximate, after all.
 			 */
-			LockBuffer(buf, BUFFER_LOCK_UNLOCK);
-			LockRelationForExtension(onerel, ExclusiveLock);
-			UnlockRelationForExtension(onerel, ExclusiveLock);
-			LockBufferForCleanup(buf);
-			if (PageIsNew(page))
-			{
-				ereport(WARNING,
-						(errmsg("relation \"%s\" page %u is uninitialized --- fixing",
-								relname, blkno)));
-				PageInit(page, BufferGetPageSize(buf), 0);
-				empty_pages++;
-			}
-			freespace = PageGetHeapFreeSpace(page);
-			MarkBufferDirty(buf);
 			UnlockReleaseBuffer(buf);
 
-			RecordPageWithFreeSpace(onerel, blkno, freespace);
+			if (GetRecordedFreeSpace(onerel, blkno) == 0)
+			{
+				Size		freespace;
+
+				freespace = BufferGetPageSize(buf) - SizeOfPageHeaderData;
+				RecordPageWithFreeSpace(onerel, blkno, freespace);
+			}
 			continue;
 		}
 
@@ -905,7 +901,10 @@ lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
 			empty_pages++;
 			freespace = PageGetHeapFreeSpace(page);
 
-			/* empty pages are always all-visible and all-frozen */
+			/*
+			 * Empty pages are always all-visible and all-frozen (note that
+			 * the same is currently not true for new pages, see above).
+			 */
 			if (!PageIsAllVisible(page))
 			{
 				START_CRIT_SECTION();
@@ -1639,12 +1638,13 @@ lazy_check_needs_freeze(Buffer buf, bool *hastup)
 
 	*hastup = false;
 
-	/* If we hit an uninitialized page, we want to force vacuuming it. */
-	if (PageIsNew(page))
-		return true;
-
-	/* Quick out for ordinary empty page. */
-	if (PageIsEmpty(page))
+	/*
+	 * New and empty pages, obviously, don't contain tuples. We could make
+	 * sure that the page is registered in the FSM, but it doesn't seem worth
+	 * waiting for a cleanup lock just for that, especially because it's
+	 * likely that the pin holder will do so.
+	 */
+	if (PageIsNew(page) || PageIsEmpty(page))
 		return false;
 
 	maxoff = PageGetMaxOffsetNumber(page);
@@ -2029,7 +2029,6 @@ count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
 
 		if (PageIsNew(page) || PageIsEmpty(page))
 		{
-			/* PageIsNew probably shouldn't happen... */
 			UnlockReleaseBuffer(buf);
 			continue;
 		}
