Ensure that WAL pages skipped by a forced WAL switch are zero-filled.

In the previous coding, skipped pages were mostly zeroes, but they still
had valid WAL page headers.  That makes them very much less compressible
than an unbroken string of zeroes would be --- about 10X worse for bzip2
compression, for instance.  We don't need those headers, so tweak the logic
so that we zero them out.

Chapman Flack, reviewed by Daniel Gustafsson

Discussion: https://postgr.es/m/579297F8.7020107@anastigmatix.net

src/backend/access/transam/xlog.c

@@ -1533,30 +1533,50 @@ CopyXLogRecordToWAL(int write_len, bool isLogSwitch, XLogRecData *rdata,
 
 	/*
 	 * If this was an xlog-switch, it's not enough to write the switch record,
-	 * we also have to consume all the remaining space in the WAL segment. We
-	 * have already reserved it for us, but we still need to make sure it's
-	 * allocated and zeroed in the WAL buffers so that when the caller (or
-	 * someone else) does XLogWrite(), it can really write out all the zeros.
+	 * we also have to consume all the remaining space in the WAL segment.  We
+	 * have already reserved that space, but we need to actually fill it.
 	 */
 	if (isLogSwitch && XLogSegmentOffset(CurrPos, wal_segment_size) != 0)
 	{
 		/* An xlog-switch record doesn't contain any data besides the header */
 		Assert(write_len == SizeOfXLogRecord);
 
-		/*
-		 * We do this one page at a time, to make sure we don't deadlock
-		 * against ourselves if wal_buffers < wal_segment_size.
-		 */
+		/* Assert that we did reserve the right amount of space */
 		Assert(XLogSegmentOffset(EndPos, wal_segment_size) == 0);
 
-		/* Use up all the remaining space on the first page */
+		/* Use up all the remaining space on the current page */
 		CurrPos += freespace;
 
+		/*
+		 * Cause all remaining pages in the segment to be flushed, leaving the
+		 * XLog position where it should be, at the start of the next segment.
+		 * We do this one page at a time, to make sure we don't deadlock
+		 * against ourselves if wal_buffers < wal_segment_size.
+		 */
 		while (CurrPos < EndPos)
 		{
-			/* initialize the next page (if not initialized already) */
-			WALInsertLockUpdateInsertingAt(CurrPos);
-			AdvanceXLInsertBuffer(CurrPos, false);
+			/*
+			 * The minimal action to flush the page would be to call
+			 * WALInsertLockUpdateInsertingAt(CurrPos) followed by
+			 * AdvanceXLInsertBuffer(...).  The page would be left initialized
+			 * mostly to zeros, except for the page header (always the short
+			 * variant, as this is never a segment's first page).
+			 *
+			 * The large vistas of zeros are good for compressibility, but the
+			 * headers interrupting them every XLOG_BLCKSZ (with values that
+			 * differ from page to page) are not.  The effect varies with
+			 * compression tool, but bzip2 for instance compresses about an
+			 * order of magnitude worse if those headers are left in place.
+			 *
+			 * Rather than complicating AdvanceXLInsertBuffer itself (which is
+			 * called in heavily-loaded circumstances as well as this lightly-
+			 * loaded one) with variant behavior, we just use GetXLogBuffer
+			 * (which itself calls the two methods we need) to get the pointer
+			 * and zero most of the page.  Then we just zero the page header.
+			 */
+			currpos = GetXLogBuffer(CurrPos);
+			MemSet(currpos, 0, SizeOfXLogShortPHD);
+
 			CurrPos += XLOG_BLCKSZ;
 		}
 	}