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, 44 insertions, 61 deletions

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index d23c292edcd..40de84e934e 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -889,14 +889,21 @@ typedef struct TwoPhaseRecordOnDisk
 
 /*
  * During prepare, the state file is assembled in memory before writing it
- * to WAL and the actual state file.  We use a chain of XLogRecData blocks
- * so that we will be able to pass the state file contents directly to
- * XLogInsert.
+ * to WAL and the actual state file.  We use a chain of StateFileChunk blocks
+ * for that.
  */
+typedef struct StateFileChunk
+{
+	char	   *data;
+	uint32		len;
+	struct StateFileChunk *next;
+} StateFileChunk;
+
 static struct xllist
 {
-	XLogRecData *head;			/* first data block in the chain */
-	XLogRecData *tail;			/* last block in chain */
+	StateFileChunk *head;		/* first data block in the chain */
+	StateFileChunk *tail;		/* last block in chain */
+	uint32		num_chunks;
 	uint32		bytes_free;		/* free bytes left in tail block */
 	uint32		total_len;		/* total data bytes in chain */
 }	records;
@@ -917,11 +924,11 @@ save_state_data(const void *data, uint32 len)
 
 	if (padlen > records.bytes_free)
 	{
-		records.tail->next = palloc0(sizeof(XLogRecData));
+		records.tail->next = palloc0(sizeof(StateFileChunk));
 		records.tail = records.tail->next;
-		records.tail->buffer = InvalidBuffer;
 		records.tail->len = 0;
 		records.tail->next = NULL;
+		records.num_chunks++;
 
 		records.bytes_free = Max(padlen, 512);
 		records.tail->data = palloc(records.bytes_free);
@@ -951,8 +958,7 @@ StartPrepare(GlobalTransaction gxact)
 	SharedInvalidationMessage *invalmsgs;
 
 	/* Initialize linked list */
-	records.head = palloc0(sizeof(XLogRecData));
-	records.head->buffer = InvalidBuffer;
+	records.head = palloc0(sizeof(StateFileChunk));
 	records.head->len = 0;
 	records.head->next = NULL;
 
@@ -960,6 +966,7 @@ StartPrepare(GlobalTransaction gxact)
 	records.head->data = palloc(records.bytes_free);
 
 	records.tail = records.head;
+	records.num_chunks = 1;
 
 	records.total_len = 0;
 
@@ -1019,7 +1026,7 @@ EndPrepare(GlobalTransaction gxact)
 	TransactionId xid = pgxact->xid;
 	TwoPhaseFileHeader *hdr;
 	char		path[MAXPGPATH];
-	XLogRecData *record;
+	StateFileChunk *record;
 	pg_crc32	statefile_crc;
 	pg_crc32	bogus_crc;
 	int			fd;
@@ -1117,12 +1124,16 @@ EndPrepare(GlobalTransaction gxact)
 	 * We save the PREPARE record's location in the gxact for later use by
 	 * CheckPointTwoPhase.
 	 */
+	XLogEnsureRecordSpace(0, records.num_chunks);
+
 	START_CRIT_SECTION();
 
 	MyPgXact->delayChkpt = true;
 
-	gxact->prepare_lsn = XLogInsert(RM_XACT_ID, XLOG_XACT_PREPARE,
-									records.head);
+	XLogBeginInsert();
+	for (record = records.head; record != NULL; record = record->next)
+		XLogRegisterData(record->data, record->len);
+	gxact->prepare_lsn = XLogInsert(RM_XACT_ID, XLOG_XACT_PREPARE);
 	XLogFlush(gxact->prepare_lsn);
 
 	/* If we crash now, we have prepared: WAL replay will fix things */
@@ -1180,6 +1191,7 @@ EndPrepare(GlobalTransaction gxact)
 	SyncRepWaitForLSN(gxact->prepare_lsn);
 
 	records.tail = records.head = NULL;
+	records.num_chunks = 0;
 }
 
 /*
@@ -2071,8 +2083,6 @@ RecordTransactionCommitPrepared(TransactionId xid,
 								SharedInvalidationMessage *invalmsgs,
 								bool initfileinval)
 {
-	XLogRecData rdata[4];
-	int			lastrdata = 0;
 	xl_xact_commit_prepared xlrec;
 	XLogRecPtr	recptr;
 
@@ -2094,39 +2104,24 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	xlrec.crec.nsubxacts = nchildren;
 	xlrec.crec.nmsgs = ninvalmsgs;
 
-	rdata[0].data = (char *) (&xlrec);
-	rdata[0].len = MinSizeOfXactCommitPrepared;
-	rdata[0].buffer = InvalidBuffer;
+	XLogBeginInsert();
+	XLogRegisterData((char *) (&xlrec), MinSizeOfXactCommitPrepared);
+
 	/* dump rels to delete */
 	if (nrels > 0)
-	{
-		rdata[0].next = &(rdata[1]);
-		rdata[1].data = (char *) rels;
-		rdata[1].len = nrels * sizeof(RelFileNode);
-		rdata[1].buffer = InvalidBuffer;
-		lastrdata = 1;
-	}
+		XLogRegisterData((char *) rels, nrels * sizeof(RelFileNode));
+
 	/* dump committed child Xids */
 	if (nchildren > 0)
-	{
-		rdata[lastrdata].next = &(rdata[2]);
-		rdata[2].data = (char *) children;
-		rdata[2].len = nchildren * sizeof(TransactionId);
-		rdata[2].buffer = InvalidBuffer;
-		lastrdata = 2;
-	}
+		XLogRegisterData((char *) children,
+						 nchildren * sizeof(TransactionId));
+
 	/* dump cache invalidation messages */
 	if (ninvalmsgs > 0)
-	{
-		rdata[lastrdata].next = &(rdata[3]);
-		rdata[3].data = (char *) invalmsgs;
-		rdata[3].len = ninvalmsgs * sizeof(SharedInvalidationMessage);
-		rdata[3].buffer = InvalidBuffer;
-		lastrdata = 3;
-	}
-	rdata[lastrdata].next = NULL;
+		XLogRegisterData((char *) invalmsgs,
+						 ninvalmsgs * sizeof(SharedInvalidationMessage));
 
-	recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_COMMIT_PREPARED, rdata);
+	recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_COMMIT_PREPARED);
 
 	/*
 	 * We don't currently try to sleep before flush here ... nor is there any
@@ -2169,8 +2164,6 @@ RecordTransactionAbortPrepared(TransactionId xid,
 							   int nrels,
 							   RelFileNode *rels)
 {
-	XLogRecData rdata[3];
-	int			lastrdata = 0;
 	xl_xact_abort_prepared xlrec;
 	XLogRecPtr	recptr;
 
@@ -2189,30 +2182,20 @@ RecordTransactionAbortPrepared(TransactionId xid,
 	xlrec.arec.xact_time = GetCurrentTimestamp();
 	xlrec.arec.nrels = nrels;
 	xlrec.arec.nsubxacts = nchildren;
-	rdata[0].data = (char *) (&xlrec);
-	rdata[0].len = MinSizeOfXactAbortPrepared;
-	rdata[0].buffer = InvalidBuffer;
+
+	XLogBeginInsert();
+	XLogRegisterData((char *) (&xlrec), MinSizeOfXactAbortPrepared);
+
 	/* dump rels to delete */
 	if (nrels > 0)
-	{
-		rdata[0].next = &(rdata[1]);
-		rdata[1].data = (char *) rels;
-		rdata[1].len = nrels * sizeof(RelFileNode);
-		rdata[1].buffer = InvalidBuffer;
-		lastrdata = 1;
-	}
+		XLogRegisterData((char *) rels, nrels * sizeof(RelFileNode));
+
 	/* dump committed child Xids */
 	if (nchildren > 0)
-	{
-		rdata[lastrdata].next = &(rdata[2]);
-		rdata[2].data = (char *) children;
-		rdata[2].len = nchildren * sizeof(TransactionId);
-		rdata[2].buffer = InvalidBuffer;
-		lastrdata = 2;
-	}
-	rdata[lastrdata].next = NULL;
+		XLogRegisterData((char *) children,
+						 nchildren * sizeof(TransactionId));
 
-	recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_ABORT_PREPARED, rdata);
+	recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_ABORT_PREPARED);
 
 	/* Always flush, since we're about to remove the 2PC state file */
 	XLogFlush(recptr);