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Commit20ba5ca

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Move WAL continuation record information to WAL page header.
The continuation record only contained one field, xl_rem_len, so it makesthings simpler to just include it in the WAL page header. This wastes fourbytes on pages that don't begin with a continuation from previos page, plusfour bytes on every page, because of padding.The motivation of this is to make it easier to calculate how much space aWAL record needs. Before this patch, it depended on how many page boundariesthe record crosses. The motivation of that, in turn, is to separate theallocation of space in the WAL from the copying of the record data to theallocated space. Keeping the calculation of space required simple helps tokeep the critical section of allocating the space from WAL short. But that'snot included in this patch yet.Bump WAL version number again, as this is an incompatible change.
1 parentdfda6eb commit20ba5ca

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2 files changed

+28
-38
lines changed

2 files changed

+28
-38
lines changed

‎src/backend/access/transam/xlog.c

Lines changed: 14 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -696,7 +696,6 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
696696
{
697697
XLogCtlInsert*Insert=&XLogCtl->Insert;
698698
XLogRecord*record;
699-
XLogContRecord*contrecord;
700699
XLogRecPtrRecPtr;
701700
XLogRecPtrWriteRqst;
702701
uint32freespace;
@@ -1085,9 +1084,7 @@ begin:;
10851084
curridx=Insert->curridx;
10861085
/* Insert cont-record header */
10871086
Insert->currpage->xlp_info |=XLP_FIRST_IS_CONTRECORD;
1088-
contrecord= (XLogContRecord*)Insert->currpos;
1089-
contrecord->xl_rem_len=write_len;
1090-
Insert->currpos+=SizeOfXLogContRecord;
1087+
Insert->currpage->xlp_rem_len=write_len;
10911088
freespace=INSERT_FREESPACE(Insert);
10921089
}
10931090

@@ -3941,7 +3938,8 @@ ReadRecord(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt)
39413938
if (total_len>len)
39423939
{
39433940
/* Need to reassemble record */
3944-
XLogContRecord*contrecord;
3941+
char*contrecord;
3942+
XLogPageHeaderpageHeader;
39453943
XLogRecPtrpagelsn;
39463944
uint32gotlen=len;
39473945

@@ -3969,39 +3967,38 @@ ReadRecord(XLogRecPtr *RecPtr, int emode, bool fetching_ckpt)
39693967
readOff)));
39703968
gotonext_record_is_invalid;
39713969
}
3972-
pageHeaderSize=XLogPageHeaderSize((XLogPageHeader)readBuf);
3973-
contrecord= (XLogContRecord*) ((char*)readBuf+pageHeaderSize);
3974-
if (contrecord->xl_rem_len==0||
3975-
total_len!= (contrecord->xl_rem_len+gotlen))
3970+
pageHeader= (XLogPageHeader)readBuf;
3971+
pageHeaderSize=XLogPageHeaderSize(pageHeader);
3972+
contrecord= (char*)readBuf+pageHeaderSize;
3973+
if (pageHeader->xlp_rem_len==0||
3974+
total_len!= (pageHeader->xlp_rem_len+gotlen))
39763975
{
39773976
charfname[MAXFNAMELEN];
39783977
XLogFileName(fname,curFileTLI,readSegNo);
39793978
ereport(emode_for_corrupt_record(emode,*RecPtr),
39803979
(errmsg("invalid contrecord length %u in log segment %s, offset %u",
3981-
contrecord->xl_rem_len,
3980+
pageHeader->xlp_rem_len,
39823981
XLogFileNameP(curFileTLI,readSegNo),
39833982
readOff)));
39843983
gotonext_record_is_invalid;
39853984
}
3986-
len=XLOG_BLCKSZ-pageHeaderSize-SizeOfXLogContRecord;
3987-
if (contrecord->xl_rem_len>len)
3985+
len=XLOG_BLCKSZ-pageHeaderSize;
3986+
if (pageHeader->xlp_rem_len>len)
39883987
{
3989-
memcpy(buffer, (char*)contrecord+SizeOfXLogContRecord,len);
3988+
memcpy(buffer, (char*)contrecord,len);
39903989
gotlen+=len;
39913990
buffer+=len;
39923991
continue;
39933992
}
3994-
memcpy(buffer, (char*)contrecord+SizeOfXLogContRecord,
3995-
contrecord->xl_rem_len);
3993+
memcpy(buffer, (char*)contrecord,pageHeader->xlp_rem_len);
39963994
break;
39973995
}
39983996
if (!RecordIsValid(record,*RecPtr,emode))
39993997
gotonext_record_is_invalid;
40003998
pageHeaderSize=XLogPageHeaderSize((XLogPageHeader)readBuf);
40013999
XLogSegNoOffsetToRecPtr(
40024000
readSegNo,
4003-
readOff+pageHeaderSize+
4004-
MAXALIGN(SizeOfXLogContRecord+contrecord->xl_rem_len),
4001+
readOff+pageHeaderSize+MAXALIGN(pageHeader->xlp_rem_len),
40054002
EndRecPtr);
40064003
ReadRecPtr=*RecPtr;
40074004
/* needn't worry about XLOG SWITCH, it can't cross page boundaries */

‎src/include/access/xlog_internal.h

Lines changed: 14 additions & 21 deletions
Original file line numberDiff line numberDiff line change
@@ -48,37 +48,30 @@ typedef struct BkpBlock
4848
/* ACTUAL BLOCK DATA FOLLOWS AT END OF STRUCT */
4949
}BkpBlock;
5050

51-
/*
52-
* When there is not enough space on current page for whole record, we
53-
* continue on the next page with continuation record.(However, the
54-
* XLogRecord header will never be split across pages; if there's less than
55-
* SizeOfXLogRecord space left at the end of a page, we just waste it.)
56-
*
57-
* Note that xl_rem_len includes backup-block data; that is, it tracks
58-
* xl_tot_len not xl_len in the initial header. Also note that the
59-
* continuation data isn't necessarily aligned.
60-
*/
61-
typedefstructXLogContRecord
62-
{
63-
uint32xl_rem_len;/* total len of remaining data for record */
64-
65-
/* ACTUAL LOG DATA FOLLOWS AT END OF STRUCT */
66-
67-
}XLogContRecord;
68-
69-
#defineSizeOfXLogContRecordsizeof(XLogContRecord)
70-
7151
/*
7252
* Each page of XLOG file has a header like this:
7353
*/
74-
#defineXLOG_PAGE_MAGIC0xD072/* can be used as WAL version indicator */
54+
#defineXLOG_PAGE_MAGIC0xD073/* can be used as WAL version indicator */
7555

7656
typedefstructXLogPageHeaderData
7757
{
7858
uint16xlp_magic;/* magic value for correctness checks */
7959
uint16xlp_info;/* flag bits, see below */
8060
TimeLineIDxlp_tli;/* TimeLineID of first record on page */
8161
XLogRecPtrxlp_pageaddr;/* XLOG address of this page */
62+
63+
/*
64+
* When there is not enough space on current page for whole record, we
65+
* continue on the next page. xlp_rem_len is the number of bytes
66+
* remaining from a previous page. (However, the XLogRecord header will
67+
* never be split across pages; if there's less than SizeOfXLogRecord
68+
* space left at the end of a page, we just waste it.)
69+
*
70+
* Note that xl_rem_len includes backup-block data; that is, it tracks
71+
* xl_tot_len not xl_len in the initial header. Also note that the
72+
* continuation data isn't necessarily aligned.
73+
*/
74+
uint32xlp_rem_len;/* total len of remaining data for record */
8275
}XLogPageHeaderData;
8376

8477
#defineSizeOfXLogShortPHDMAXALIGN(sizeof(XLogPageHeaderData))

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