Revamp the WAL record format.

Each WAL record now carries information about the modified relation and
block(s) in a standardized format. That makes it easier to write tools that
need that information, like pg_rewind, prefetching the blocks to speed up
recovery, etc.

There's a whole new API for building WAL records, replacing the XLogRecData
chains used previously. The new API consists of XLogRegister* functions,
which are called for each buffer and chunk of data that is added to the
record. The new API also gives more control over when a full-page image is
written, by passing flags to the XLogRegisterBuffer function.

This also simplifies the XLogReadBufferForRedo() calls. The function can dig
the relation and block number from the WAL record, so they no longer need to
be passed as arguments.

For the convenience of redo routines, XLogReader now disects each WAL record
after reading it, copying the main data part and the per-block data into
MAXALIGNed buffers. The data chunks are not aligned within the WAL record,
but the redo routines can assume that the pointers returned by XLogRecGet*
functions are. Redo routines are now passed the XLogReaderState, which
contains the record in the already-disected format, instead of the plain
XLogRecord.

The new record format also makes the fixed size XLogRecord header smaller,
by removing the xl_len field. The length of the "main data" portion is now
stored at the end of the WAL record, and there's a separate header after
XLogRecord for it. The alignment padding at the end of XLogRecord is also
removed. This compansates for the fact that the new format would otherwise
be more bulky than the old format.

Reviewed by Andres Freund, Amit Kapila, Michael Paquier, Alvaro Herrera,
Fujii Masao.

1 files changed, 59 insertions, 52 deletions

diff --git a/src/backend/access/gin/ginbtree.c b/src/backend/access/gin/ginbtree.c
index 5365477000a..99f40a871f0 100644
--- a/src/backend/access/gin/ginbtree.c
+++ b/src/backend/access/gin/ginbtree.c
@@ -326,7 +326,6 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 			   Buffer childbuf, GinStatsData *buildStats)
 {
 	Page		page = BufferGetPage(stack->buffer);
-	XLogRecData *payloadrdata;
 	GinPlaceToPageRC rc;
 	uint16		xlflags = 0;
 	Page		childpage = NULL;
@@ -351,12 +350,36 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 	/*
 	 * Try to put the incoming tuple on the page. placeToPage will decide if
 	 * the page needs to be split.
+	 *
+	 * WAL-logging this operation is a bit funny:
+	 *
+	 * We're responsible for calling XLogBeginInsert() and XLogInsert().
+	 * XLogBeginInsert() must be called before placeToPage, because
+	 * placeToPage can register some data to the WAL record.
+	 *
+	 * If placeToPage returns INSERTED, placeToPage has already called
+	 * START_CRIT_SECTION(), and we're responsible for calling
+	 * END_CRIT_SECTION. When it returns INSERTED, it is also responsible for
+	 * registering any data required to replay the operation with
+	 * XLogRegisterData(0, ...). It may only add data to block index 0; the
+	 * main data of the WAL record is reserved for this function.
+	 *
+	 * If placeToPage returns SPLIT, we're wholly responsible for WAL logging.
+	 * Splits happen infrequently, so we just make a full-page image of all
+	 * the pages involved.
 	 */
+
+	if (RelationNeedsWAL(btree->index))
+		XLogBeginInsert();
+
 	rc = btree->placeToPage(btree, stack->buffer, stack,
 							insertdata, updateblkno,
-							&payloadrdata, &newlpage, &newrpage);
+							&newlpage, &newrpage);
 	if (rc == UNMODIFIED)
+	{
+		XLogResetInsertion();
 		return true;
+	}
 	else if (rc == INSERTED)
 	{
 		/* placeToPage did START_CRIT_SECTION() */
@@ -372,17 +395,18 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		if (RelationNeedsWAL(btree->index))
 		{
 			XLogRecPtr	recptr;
-			XLogRecData rdata[3];
 			ginxlogInsert xlrec;
 			BlockIdData childblknos[2];
 
-			xlrec.node = btree->index->rd_node;
-			xlrec.blkno = BufferGetBlockNumber(stack->buffer);
+			/*
+			 * placetopage already registered stack->buffer as block 0.
+			 */
 			xlrec.flags = xlflags;
 
-			rdata[0].buffer = InvalidBuffer;
-			rdata[0].data = (char *) &xlrec;
-			rdata[0].len = sizeof(ginxlogInsert);
+			if (childbuf != InvalidBuffer)
+				XLogRegisterBuffer(1, childbuf, REGBUF_STANDARD);
+
+			XLogRegisterData((char *) &xlrec, sizeof(ginxlogInsert));
 
 			/*
 			 * Log information about child if this was an insertion of a
@@ -390,26 +414,13 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 			 */
 			if (childbuf != InvalidBuffer)
 			{
-				rdata[0].next = &rdata[1];
-
 				BlockIdSet(&childblknos[0], BufferGetBlockNumber(childbuf));
 				BlockIdSet(&childblknos[1], GinPageGetOpaque(childpage)->rightlink);
-
-				rdata[1].buffer = InvalidBuffer;
-				rdata[1].data = (char *) childblknos;
-				rdata[1].len = sizeof(BlockIdData) * 2;
-				rdata[1].next = &rdata[2];
-
-				rdata[2].buffer = childbuf;
-				rdata[2].buffer_std = false;
-				rdata[2].data = NULL;
-				rdata[2].len = 0;
-				rdata[2].next = payloadrdata;
+				XLogRegisterData((char *) childblknos,
+								 sizeof(BlockIdData) * 2);
 			}
-			else
-				rdata[0].next = payloadrdata;
 
-			recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_INSERT, rdata);
+			recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_INSERT);
 			PageSetLSN(page, recptr);
 			if (childbuf != InvalidBuffer)
 				PageSetLSN(childpage, recptr);
@@ -421,10 +432,9 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 	}
 	else if (rc == SPLIT)
 	{
-		/* Didn't fit, have to split */
+		/* Didn't fit, had to split */
 		Buffer		rbuffer;
 		BlockNumber savedRightLink;
-		XLogRecData rdata[2];
 		ginxlogSplit data;
 		Buffer		lbuffer = InvalidBuffer;
 		Page		newrootpg = NULL;
@@ -448,7 +458,6 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		 */
 
 		data.node = btree->index->rd_node;
-		data.rblkno = BufferGetBlockNumber(rbuffer);
 		data.flags = xlflags;
 		if (childbuf != InvalidBuffer)
 		{
@@ -462,23 +471,6 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		else
 			data.leftChildBlkno = data.rightChildBlkno = InvalidBlockNumber;
 
-		rdata[0].buffer = InvalidBuffer;
-		rdata[0].data = (char *) &data;
-		rdata[0].len = sizeof(ginxlogSplit);
-
-		if (childbuf != InvalidBuffer)
-		{
-			rdata[0].next = &rdata[1];
-
-			rdata[1].buffer = childbuf;
-			rdata[1].buffer_std = false;
-			rdata[1].data = NULL;
-			rdata[1].len = 0;
-			rdata[1].next = payloadrdata;
-		}
-		else
-			rdata[0].next = payloadrdata;
-
 		if (stack->parent == NULL)
 		{
 			/*
@@ -496,12 +488,7 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 					buildStats->nEntryPages++;
 			}
 
-			/*
-			 * root never has a right-link, so we borrow the rrlink field to
-			 * store the root block number.
-			 */
-			data.rrlink = BufferGetBlockNumber(stack->buffer);
-			data.lblkno = BufferGetBlockNumber(lbuffer);
+			data.rrlink = InvalidBlockNumber;
 			data.flags |= GIN_SPLIT_ROOT;
 
 			GinPageGetOpaque(newrpage)->rightlink = InvalidBlockNumber;
@@ -524,7 +511,6 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		{
 			/* split non-root page */
 			data.rrlink = savedRightLink;
-			data.lblkno = BufferGetBlockNumber(stack->buffer);
 
 			GinPageGetOpaque(newrpage)->rightlink = savedRightLink;
 			GinPageGetOpaque(newlpage)->flags |= GIN_INCOMPLETE_SPLIT;
@@ -572,7 +558,28 @@ ginPlaceToPage(GinBtree btree, GinBtreeStack *stack,
 		{
 			XLogRecPtr	recptr;
 
-			recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_SPLIT, rdata);
+			/*
+			 * We just take full page images of all the split pages. Splits
+			 * are uncommon enough that it's not worth complicating the code
+			 * to be more efficient.
+			 */
+			if (stack->parent == NULL)
+			{
+				XLogRegisterBuffer(0, lbuffer, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
+				XLogRegisterBuffer(1, rbuffer, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
+				XLogRegisterBuffer(2, stack->buffer, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
+			}
+			else
+			{
+				XLogRegisterBuffer(0, stack->buffer, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
+				XLogRegisterBuffer(1, rbuffer, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
+			}
+			if (BufferIsValid(childbuf))
+				XLogRegisterBuffer(3, childbuf, 0);
+
+			XLogRegisterData((char *) &data, sizeof(ginxlogSplit));
+
+			recptr = XLogInsert(RM_GIN_ID, XLOG_GIN_SPLIT);
 			PageSetLSN(BufferGetPage(stack->buffer), recptr);
 			PageSetLSN(BufferGetPage(rbuffer), recptr);
 			if (stack->parent == NULL)