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, 6 insertions, 4 deletions

diff --git a/src/include/access/xlog.h b/src/include/access/xlog.h
index 6f8b5f46e10..d06fbc0ec1e 100644
--- a/src/include/access/xlog.h
+++ b/src/include/access/xlog.h
@@ -14,7 +14,7 @@
 #include "access/rmgr.h"
 #include "access/xlogdefs.h"
 #include "access/xloginsert.h"
-#include "access/xlogrecord.h"
+#include "access/xlogreader.h"
 #include "datatype/timestamp.h"
 #include "lib/stringinfo.h"
 
@@ -186,7 +186,9 @@ typedef struct CheckpointStatsData
 
 extern CheckpointStatsData CheckpointStats;
 
-extern XLogRecPtr XLogInsertRecord(XLogRecData *rdata, XLogRecPtr fpw_lsn);
+struct XLogRecData;
+
+extern XLogRecPtr XLogInsertRecord(struct XLogRecData *rdata, XLogRecPtr fpw_lsn);
 extern void XLogFlush(XLogRecPtr RecPtr);
 extern bool XLogBackgroundFlush(void);
 extern bool XLogNeedsFlush(XLogRecPtr RecPtr);
@@ -198,8 +200,8 @@ extern XLogSegNo XLogGetLastRemovedSegno(void);
 extern void XLogSetAsyncXactLSN(XLogRecPtr record);
 extern void XLogSetReplicationSlotMinimumLSN(XLogRecPtr lsn);
 
-extern void xlog_redo(XLogRecPtr lsn, XLogRecord *record);
-extern void xlog_desc(StringInfo buf, XLogRecord *record);
+extern void xlog_redo(XLogReaderState *record);
+extern void xlog_desc(StringInfo buf, XLogReaderState *record);
 extern const char *xlog_identify(uint8 info);
 
 extern void issue_xlog_fsync(int fd, XLogSegNo segno);