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Commit0a87394

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Fix the torn-page hazard for PITR base backups by forcing full page writes
to occur between pg_start_backup() and pg_stop_backup(), even if the GUCsetting full_page_writes is OFF. Per discussion, doing this in combinationwith the already-existing checkpoint during pg_start_backup() should ensuresafety against partial page updates being included in the backup. We donot have to force full page writes to occur during normal PITR operation,as I had first feared.
1 parent8e7aaeb commit0a87394

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1 file changed

+156
-78
lines changed
  • src/backend/access/transam

1 file changed

+156
-78
lines changed

‎src/backend/access/transam/xlog.c

Lines changed: 156 additions & 78 deletions
Original file line numberDiff line numberDiff line change
@@ -7,7 +7,7 @@
77
* Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
88
* Portions Copyright (c) 1994, Regents of the University of California
99
*
10-
* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.235 2006/04/14 20:27:24 tgl Exp $
10+
* $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.236 2006/04/17 18:55:05 tgl Exp $
1111
*
1212
*-------------------------------------------------------------------------
1313
*/
@@ -344,6 +344,7 @@ typedef struct XLogCtlInsert
344344
XLogPageHeadercurrpage;/* points to header of block in cache */
345345
char*currpos;/* current insertion point in cache */
346346
XLogRecPtrRedoRecPtr;/* current redo point for insertions */
347+
boolforcePageWrites;/* forcing full-page writes for PITR? */
347348
}XLogCtlInsert;
348349

349350
/*
@@ -466,7 +467,7 @@ static void exitArchiveRecovery(TimeLineID endTLI,
466467
uint32endLogId,uint32endLogSeg);
467468
staticboolrecoveryStopsHere(XLogRecord*record,bool*includeThis);
468469

469-
staticboolXLogCheckBuffer(XLogRecData*rdata,
470+
staticboolXLogCheckBuffer(XLogRecData*rdata,booldoPageWrites,
470471
XLogRecPtr*lsn,BkpBlock*bkpb);
471472
staticboolAdvanceXLInsertBuffer(void);
472473
staticvoidXLogWrite(XLogwrtRqstWriteRqst,boolflexible);
@@ -544,6 +545,7 @@ XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
544545
unsignedi;
545546
XLogwrtRqstLogwrtRqst;
546547
boolupdrqst;
548+
booldoPageWrites;
547549
boolno_tran= (rmid==RM_XLOG_ID) ? true : false;
548550

549551
if (info&XLR_INFO_MASK)
@@ -591,6 +593,14 @@ begin:;
591593
dtbuf_bkp[i]= false;
592594
}
593595

596+
/*
597+
* Decide if we need to do full-page writes in this XLOG record: true if
598+
* full_page_writes is on or we have a PITR request for it. Since we
599+
* don't yet have the insert lock, forcePageWrites could change under us,
600+
* but we'll recheck it once we have the lock.
601+
*/
602+
doPageWrites=fullPageWrites||Insert->forcePageWrites;
603+
594604
INIT_CRC32(rdata_crc);
595605
len=0;
596606
for (rdt=rdata;;)
@@ -622,7 +632,8 @@ begin:;
622632
{
623633
/* OK, put it in this slot */
624634
dtbuf[i]=rdt->buffer;
625-
if (XLogCheckBuffer(rdt,&(dtbuf_lsn[i]),&(dtbuf_xlg[i])))
635+
if (XLogCheckBuffer(rdt,doPageWrites,
636+
&(dtbuf_lsn[i]),&(dtbuf_xlg[i])))
626637
{
627638
dtbuf_bkp[i]= true;
628639
rdt->data=NULL;
@@ -735,30 +746,51 @@ begin:;
735746
* Check to see if my RedoRecPtr is out of date. If so, may have to go
736747
* back and recompute everything. This can only happen just after a
737748
* checkpoint, so it's better to be slow in this case and fast otherwise.
749+
*
750+
* If we aren't doing full-page writes then RedoRecPtr doesn't actually
751+
* affect the contents of the XLOG record, so we'll update our local
752+
* copy but not force a recomputation.
738753
*/
739754
if (!XLByteEQ(RedoRecPtr,Insert->RedoRecPtr))
740755
{
741756
Assert(XLByteLT(RedoRecPtr,Insert->RedoRecPtr));
742757
RedoRecPtr=Insert->RedoRecPtr;
743758

744-
for (i=0;i<XLR_MAX_BKP_BLOCKS;i++)
759+
if (doPageWrites)
745760
{
746-
if (dtbuf[i]==InvalidBuffer)
747-
continue;
748-
if (dtbuf_bkp[i]== false&&
749-
XLByteLE(dtbuf_lsn[i],RedoRecPtr))
761+
for (i=0;i<XLR_MAX_BKP_BLOCKS;i++)
750762
{
751-
/*
752-
* Oops, this buffer now needs to be backed up, but we didn't
753-
* think so above.Start over.
754-
*/
755-
LWLockRelease(WALInsertLock);
756-
END_CRIT_SECTION();
757-
gotobegin;
763+
if (dtbuf[i]==InvalidBuffer)
764+
continue;
765+
if (dtbuf_bkp[i]== false&&
766+
XLByteLE(dtbuf_lsn[i],RedoRecPtr))
767+
{
768+
/*
769+
* Oops, this buffer now needs to be backed up, but we
770+
* didn't think so above. Start over.
771+
*/
772+
LWLockRelease(WALInsertLock);
773+
END_CRIT_SECTION();
774+
gotobegin;
775+
}
758776
}
759777
}
760778
}
761779

780+
/*
781+
* Also check to see if forcePageWrites was just turned on; if we
782+
* weren't already doing full-page writes then go back and recompute.
783+
* (If it was just turned off, we could recompute the record without
784+
* full pages, but we choose not to bother.)
785+
*/
786+
if (Insert->forcePageWrites&& !doPageWrites)
787+
{
788+
/* Oops, must redo it with full-page data */
789+
LWLockRelease(WALInsertLock);
790+
END_CRIT_SECTION();
791+
gotobegin;
792+
}
793+
762794
/*
763795
* Make additional rdata chain entries for the backup blocks, so that we
764796
* don't need to special-case them in the write loop. Note that we have
@@ -966,7 +998,7 @@ begin:;
966998
* save the buffer's LSN at *lsn.
967999
*/
9681000
staticbool
969-
XLogCheckBuffer(XLogRecData*rdata,
1001+
XLogCheckBuffer(XLogRecData*rdata,booldoPageWrites,
9701002
XLogRecPtr*lsn,BkpBlock*bkpb)
9711003
{
9721004
PageHeaderpage;
@@ -980,7 +1012,7 @@ XLogCheckBuffer(XLogRecData *rdata,
9801012
*/
9811013
*lsn=page->pd_lsn;
9821014

983-
if (fullPageWrites&&
1015+
if (doPageWrites&&
9841016
XLByteLE(page->pd_lsn,RedoRecPtr))
9851017
{
9861018
/*
@@ -5651,76 +5683,120 @@ pg_start_backup(PG_FUNCTION_ARGS)
56515683
PointerGetDatum(backupid)));
56525684

56535685
/*
5654-
* Force a CHECKPOINT.This is not strictly necessary, but it seems like
5655-
* a good idea to minimize the amount of past WAL needed to use the
5656-
* backup.Also, this guarantees that two successive backup runs will
5657-
* have different checkpoint positions and hence different history file
5658-
* names, even if nothing happened in between.
5686+
* Mark backup active in shared memory. We must do full-page WAL writes
5687+
* during an on-line backup even if not doing so at other times, because
5688+
* it's quite possible for the backup dump to obtain a "torn" (partially
5689+
* written) copy of a database page if it reads the page concurrently
5690+
* with our write to the same page. This can be fixed as long as the
5691+
* first write to the page in the WAL sequence is a full-page write.
5692+
* Hence, we turn on forcePageWrites and then force a CHECKPOINT, to
5693+
* ensure there are no dirty pages in shared memory that might get
5694+
* dumped while the backup is in progress without having a corresponding
5695+
* WAL record. (Once the backup is complete, we need not force full-page
5696+
* writes anymore, since we expect that any pages not modified during
5697+
* the backup interval must have been correctly captured by the backup.)
5698+
*
5699+
* We must hold WALInsertLock to change the value of forcePageWrites,
5700+
* to ensure adequate interlocking against XLogInsert().
56595701
*/
5660-
RequestCheckpoint(true, false);
5702+
LWLockAcquire(WALInsertLock,LW_EXCLUSIVE);
5703+
if (XLogCtl->Insert.forcePageWrites)
5704+
{
5705+
LWLockRelease(WALInsertLock);
5706+
ereport(ERROR,
5707+
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
5708+
errmsg("a backup is already in progress"),
5709+
errhint("Run pg_stop_backup() and try again.")));
5710+
}
5711+
XLogCtl->Insert.forcePageWrites= true;
5712+
LWLockRelease(WALInsertLock);
56615713

5662-
/*
5663-
* Now we need to fetch the checkpoint record location, and also its REDO
5664-
* pointer. The oldest point in WAL that would be needed to restore
5665-
* starting from the checkpoint is precisely the REDO pointer.
5666-
*/
5667-
LWLockAcquire(ControlFileLock,LW_EXCLUSIVE);
5668-
checkpointloc=ControlFile->checkPoint;
5669-
startpoint=ControlFile->checkPointCopy.redo;
5670-
LWLockRelease(ControlFileLock);
5714+
/* Use a TRY block to ensure we release forcePageWrites if fail below */
5715+
PG_TRY();
5716+
{
5717+
/*
5718+
* Force a CHECKPOINT. Aside from being necessary to prevent torn
5719+
* page problems, this guarantees that two successive backup runs will
5720+
* have different checkpoint positions and hence different history
5721+
* file names, even if nothing happened in between.
5722+
*/
5723+
RequestCheckpoint(true, false);
56715724

5672-
XLByteToSeg(startpoint,_logId,_logSeg);
5673-
XLogFileName(xlogfilename,ThisTimeLineID,_logId,_logSeg);
5725+
/*
5726+
* Now we need to fetch the checkpoint record location, and also its
5727+
* REDO pointer. The oldest point in WAL that would be needed to
5728+
* restore starting from the checkpoint is precisely the REDO pointer.
5729+
*/
5730+
LWLockAcquire(ControlFileLock,LW_EXCLUSIVE);
5731+
checkpointloc=ControlFile->checkPoint;
5732+
startpoint=ControlFile->checkPointCopy.redo;
5733+
LWLockRelease(ControlFileLock);
56745734

5675-
/*
5676-
* We deliberately use strftime/localtime not the src/timezone functions,
5677-
* so that backup labels will consistently be recorded in the same
5678-
* timezone regardless of TimeZone setting. This matches elog.c's
5679-
* practice.
5680-
*/
5681-
stamp_time=time(NULL);
5682-
strftime(strfbuf,sizeof(strfbuf),
5683-
"%Y-%m-%d %H:%M:%S %Z",
5684-
localtime(&stamp_time));
5735+
XLByteToSeg(startpoint,_logId,_logSeg);
5736+
XLogFileName(xlogfilename,ThisTimeLineID,_logId,_logSeg);
56855737

5686-
/*
5687-
* Check for existing backup label --- implies a backup is already running
5688-
*/
5689-
if (stat(BACKUP_LABEL_FILE,&stat_buf)!=0)
5690-
{
5691-
if (errno!=ENOENT)
5738+
/*
5739+
* We deliberately use strftime/localtime not the src/timezone
5740+
* functions, so that backup labels will consistently be recorded in
5741+
* the same timezone regardless of TimeZone setting. This matches
5742+
* elog.c's practice.
5743+
*/
5744+
stamp_time=time(NULL);
5745+
strftime(strfbuf,sizeof(strfbuf),
5746+
"%Y-%m-%d %H:%M:%S %Z",
5747+
localtime(&stamp_time));
5748+
5749+
/*
5750+
* Check for existing backup label --- implies a backup is already
5751+
* running. (XXX given that we checked forcePageWrites above, maybe
5752+
* it would be OK to just unlink any such label file?)
5753+
*/
5754+
if (stat(BACKUP_LABEL_FILE,&stat_buf)!=0)
5755+
{
5756+
if (errno!=ENOENT)
5757+
ereport(ERROR,
5758+
(errcode_for_file_access(),
5759+
errmsg("could not stat file \"%s\": %m",
5760+
BACKUP_LABEL_FILE)));
5761+
}
5762+
else
5763+
ereport(ERROR,
5764+
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
5765+
errmsg("a backup is already in progress"),
5766+
errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
5767+
BACKUP_LABEL_FILE)));
5768+
5769+
/*
5770+
* Okay, write the file
5771+
*/
5772+
fp=AllocateFile(BACKUP_LABEL_FILE,"w");
5773+
if (!fp)
56925774
ereport(ERROR,
56935775
(errcode_for_file_access(),
5694-
errmsg("could not stat file \"%s\": %m",
5776+
errmsg("could not create file \"%s\": %m",
5777+
BACKUP_LABEL_FILE)));
5778+
fprintf(fp,"START WAL LOCATION: %X/%X (file %s)\n",
5779+
startpoint.xlogid,startpoint.xrecoff,xlogfilename);
5780+
fprintf(fp,"CHECKPOINT LOCATION: %X/%X\n",
5781+
checkpointloc.xlogid,checkpointloc.xrecoff);
5782+
fprintf(fp,"START TIME: %s\n",strfbuf);
5783+
fprintf(fp,"LABEL: %s\n",backupidstr);
5784+
if (fflush(fp)||ferror(fp)||FreeFile(fp))
5785+
ereport(ERROR,
5786+
(errcode_for_file_access(),
5787+
errmsg("could not write file \"%s\": %m",
56955788
BACKUP_LABEL_FILE)));
56965789
}
5697-
else
5698-
ereport(ERROR,
5699-
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
5700-
errmsg("a backup is already in progress"),
5701-
errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
5702-
BACKUP_LABEL_FILE)));
5790+
PG_CATCH();
5791+
{
5792+
/* Turn off forcePageWrites on failure */
5793+
LWLockAcquire(WALInsertLock,LW_EXCLUSIVE);
5794+
XLogCtl->Insert.forcePageWrites= false;
5795+
LWLockRelease(WALInsertLock);
57035796

5704-
/*
5705-
* Okay, write the file
5706-
*/
5707-
fp=AllocateFile(BACKUP_LABEL_FILE,"w");
5708-
if (!fp)
5709-
ereport(ERROR,
5710-
(errcode_for_file_access(),
5711-
errmsg("could not create file \"%s\": %m",
5712-
BACKUP_LABEL_FILE)));
5713-
fprintf(fp,"START WAL LOCATION: %X/%X (file %s)\n",
5714-
startpoint.xlogid,startpoint.xrecoff,xlogfilename);
5715-
fprintf(fp,"CHECKPOINT LOCATION: %X/%X\n",
5716-
checkpointloc.xlogid,checkpointloc.xrecoff);
5717-
fprintf(fp,"START TIME: %s\n",strfbuf);
5718-
fprintf(fp,"LABEL: %s\n",backupidstr);
5719-
if (fflush(fp)||ferror(fp)||FreeFile(fp))
5720-
ereport(ERROR,
5721-
(errcode_for_file_access(),
5722-
errmsg("could not write file \"%s\": %m",
5723-
BACKUP_LABEL_FILE)));
5797+
PG_RE_THROW();
5798+
}
5799+
PG_END_TRY();
57245800

57255801
/*
57265802
* We're done. As a convenience, return the starting WAL offset.
@@ -5766,10 +5842,12 @@ pg_stop_backup(PG_FUNCTION_ARGS)
57665842

57675843
/*
57685844
* Get the current end-of-WAL position; it will be unsafe to use this dump
5769-
* to restore to a point in advance of this time.
5845+
* to restore to a point in advance of this time. We can also clear
5846+
* forcePageWrites here.
57705847
*/
57715848
LWLockAcquire(WALInsertLock,LW_EXCLUSIVE);
57725849
INSERT_RECPTR(stoppoint,Insert,Insert->curridx);
5850+
XLogCtl->Insert.forcePageWrites= false;
57735851
LWLockRelease(WALInsertLock);
57745852

57755853
XLByteToSeg(stoppoint,_logId,_logSeg);

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