Modify XLogInsert API to make callers specify whether pages to be backed

up have the standard layout with unused space between pd_lower and pd_upper.
When this is set, XLogInsert will omit the unused space without bothering
to scan it to see if it's zero.  That saves time in XLogInsert, and also
allows reversion of my earlier patch to make PageRepairFragmentation et al
explicitly re-zero freed space.  Per suggestion by Heikki Linnakangas.

1 files changed, 29 insertions, 11 deletions

diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 554c3c3aec7..7547d7f5b9f 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.63 2005/06/06 17:01:24 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/access/xlog.h,v 1.64 2005/06/06 20:22:58 tgl Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -91,24 +91,42 @@ typedef struct XLogRecord
 extern int	sync_method;
 
 /*
- * List of these structs is used to pass data to XLogInsert().
+ * The rmgr data to be written by XLogInsert() is defined by a chain of
+ * one or more XLogRecData structs.  (Multiple structs would be used when
+ * parts of the source data aren't physically adjacent in memory, or when
+ * multiple associated buffers need to be specified.)
  *
  * If buffer is valid then XLOG will check if buffer must be backed up
  * (ie, whether this is first change of that page since last checkpoint).
  * If so, the whole page contents are attached to the XLOG record, and XLOG
  * sets XLR_BKP_BLOCK_X bit in xl_info.  Note that the buffer must be pinned
- * and locked while this is going on, so that it won't change under us.
- * NB: when this happens, we do not bother to insert the associated data into
- * the XLOG record, since we assume it's present in the buffer.  Therefore,
- * rmgr redo routines MUST pay attention to XLR_BKP_BLOCK_X to know what
- * is actually stored in the XLOG record.
+ * and exclusive-locked by the caller, so that it won't change under us.
+ * NB: when the buffer is backed up, we DO NOT insert the data pointed to by
+ * this XLogRecData struct into the XLOG record, since we assume it's present
+ * in the buffer.  Therefore, rmgr redo routines MUST pay attention to
+ * XLR_BKP_BLOCK_X to know what is actually stored in the XLOG record.
+ * The i'th XLR_BKP_BLOCK bit corresponds to the i'th distinct buffer
+ * value (ignoring InvalidBuffer) appearing in the rdata chain.
+ *
+ * When buffer is valid, caller must set buffer_std to indicate whether the
+ * page uses standard pd_lower/pd_upper header fields.  If this is true, then
+ * XLOG is allowed to omit the free space between pd_lower and pd_upper from
+ * the backed-up page image.  Note that even when buffer_std is false, the
+ * page MUST have an LSN field as its first eight bytes!
+ *
+ * Note: data can be NULL to indicate no rmgr data associated with this chain
+ * entry.  This can be sensible (ie, not a wasted entry) if buffer is valid.
+ * The implication is that the buffer has been changed by the operation being
+ * logged, and so may need to be backed up, but the change can be redone using
+ * only information already present elsewhere in the XLOG entry.
  */
 typedef struct XLogRecData
 {
-	Buffer		buffer;			/* buffer associated with this data */
-	char	   *data;
-	uint32		len;
-	struct XLogRecData *next;
+	char	   *data;			/* start of rmgr data to include */
+	uint32		len;			/* length of rmgr data to include */
+	Buffer		buffer;			/* buffer associated with data, if any */
+	bool		buffer_std;		/* buffer has standard pd_lower/pd_upper */
+	struct XLogRecData *next;	/* next struct in chain, or NULL */
 } XLogRecData;
 
 extern TimeLineID ThisTimeLineID;		/* current TLI */