CreateCheckPoint(int flags)
{
	/* use volatile pointer to prevent code rearrangement */
	volatile XLogCtlData *xlogctl = XLogCtl;
	bool		shutdown;
	CheckPoint	checkPoint;
	XLogRecPtr	recptr;
	XLogCtlInsert *Insert = &XLogCtl->Insert;
	XLogRecData rdata;
	uint32		freespace;
	XLogSegNo	_logSegNo;
	XLogRecPtr	curInsert;
	VirtualTransactionId *vxids;
	int			nvxids;

	/*
	 * An end-of-recovery checkpoint is really a shutdown checkpoint, just
	 * issued at a different time.
	 */
	if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
		shutdown = true;
	else
		shutdown = false;

	/* sanity check */
	if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
		elog(ERROR, "can't create a checkpoint during recovery");

	/*
	 * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
	 * (This is just pro forma, since in the present system structure there is
	 * only one process that is allowed to issue checkpoints at any given
	 * time.)
	 */
	LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

	/*
	 * Prepare to accumulate statistics.
	 *
	 * Note: because it is possible for log_checkpoints to change while a
	 * checkpoint proceeds, we always accumulate stats, even if
	 * log_checkpoints is currently off.
	 */
	MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
	CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

	/*
	 * Use a critical section to force system panic if we have trouble.
	 */
	START_CRIT_SECTION();

	if (shutdown)
	{
		LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
		ControlFile->state = DB_SHUTDOWNING;
		ControlFile->time = (pg_time_t) time(NULL);
		UpdateControlFile();
		LWLockRelease(ControlFileLock);
	}

	/*
	 * Let smgr prepare for checkpoint; this has to happen before we determine
	 * the REDO pointer.  Note that smgr must not do anything that'd have to
	 * be undone if we decide no checkpoint is needed.
	 */
	smgrpreckpt();

	/* Begin filling in the checkpoint WAL record */
	MemSet(&checkPoint, 0, sizeof(checkPoint));
	checkPoint.time = (pg_time_t) time(NULL);

	/*
	 * For Hot Standby, derive the oldestActiveXid before we fix the redo
	 * pointer. This allows us to begin accumulating changes to assemble our
	 * starting snapshot of locks and transactions.
	 */
	if (!shutdown && XLogStandbyInfoActive())
		checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
	else
		checkPoint.oldestActiveXid = InvalidTransactionId;

	/*
	 * We must block concurrent insertions while examining insert state to
	 * determine the checkpoint REDO pointer.
	 */
	WALInsertSlotAcquire(true);
	curInsert = XLogBytePosToRecPtr(Insert->CurrBytePos);

	/*
	 * If this isn't a shutdown or forced checkpoint, and we have not inserted
	 * any XLOG records since the start of the last checkpoint, skip the
	 * checkpoint.	The idea here is to avoid inserting duplicate checkpoints
	 * when the system is idle. That wastes log space, and more importantly it
	 * exposes us to possible loss of both current and previous checkpoint
	 * records if the machine crashes just as we're writing the update.
	 * (Perhaps it'd make even more sense to checkpoint only when the previous
	 * checkpoint record is in a different xlog page?)
	 *
	 * We have to make two tests to determine that nothing has happened since
	 * the start of the last checkpoint: current insertion point must match
	 * the end of the last checkpoint record, and its redo pointer must point
	 * to itself.
	 */
	if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
				  CHECKPOINT_FORCE)) == 0)
	{
		if (curInsert == ControlFile->checkPoint +
			MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
			ControlFile->checkPoint == ControlFile->checkPointCopy.redo)
		{
			WALInsertSlotRelease();
			LWLockRelease(CheckpointLock);
			END_CRIT_SECTION();
			return;
		}
	}

	/*
	 * An end-of-recovery checkpoint is created before anyone is allowed to
	 * write WAL. To allow us to write the checkpoint record, temporarily
	 * enable XLogInsertAllowed.  (This also ensures ThisTimeLineID is
	 * initialized, which we need here and in AdvanceXLInsertBuffer.)
	 */
	if (flags & CHECKPOINT_END_OF_RECOVERY)
		LocalSetXLogInsertAllowed();

	checkPoint.ThisTimeLineID = ThisTimeLineID;
	if (flags & CHECKPOINT_END_OF_RECOVERY)
		checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
	else
		checkPoint.PrevTimeLineID = ThisTimeLineID;

	checkPoint.fullPageWrites = Insert->fullPageWrites;

	/*
	 * Compute new REDO record ptr = location of next XLOG record.
	 *
	 * NB: this is NOT necessarily where the checkpoint record itself will be,
	 * since other backends may insert more XLOG records while we're off doing
	 * the buffer flush work.  Those XLOG records are logically after the
	 * checkpoint, even though physically before it.  Got that?
	 */
	freespace = INSERT_FREESPACE(curInsert);
	if (freespace == 0)
	{
		if (curInsert % XLogSegSize == 0)
			curInsert += SizeOfXLogLongPHD;
		else
			curInsert += SizeOfXLogShortPHD;
	}
	checkPoint.redo = curInsert;

	/*
	 * Here we update the shared RedoRecPtr for future XLogInsert calls; this
	 * must be done while holding the insertion slots.
	 *
	 * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
	 * pointing past where it really needs to point.  This is okay; the only
	 * consequence is that XLogInsert might back up whole buffers that it
	 * didn't really need to.  We can't postpone advancing RedoRecPtr because
	 * XLogInserts that happen while we are dumping buffers must assume that
	 * their buffer changes are not included in the checkpoint.
	 */
	RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;

	/*
	 * Now we can release the WAL insertion slots, allowing other xacts to
	 * proceed while we are flushing disk buffers.
	 */
	WALInsertSlotRelease();

	/* Update the info_lck-protected copy of RedoRecPtr as well */
	SpinLockAcquire(&xlogctl->info_lck);
	xlogctl->RedoRecPtr = checkPoint.redo;
	SpinLockRelease(&xlogctl->info_lck);

	/*
	 * If enabled, log checkpoint start.  We postpone this until now so as not
	 * to log anything if we decided to skip the checkpoint.
	 */
	if (log_checkpoints)
		LogCheckpointStart(flags, false);

	TRACE_POSTGRESQL_CHECKPOINT_START(flags);

	/*
	 * In some cases there are groups of actions that must all occur on one
	 * side or the other of a checkpoint record. Before flushing the
	 * checkpoint record we must explicitly wait for any backend currently
	 * performing those groups of actions.
	 *
	 * One example is end of transaction, so we must wait for any transactions
	 * that are currently in commit critical sections.	If an xact inserted
	 * its commit record into XLOG just before the REDO point, then a crash
	 * restart from the REDO point would not replay that record, which means
	 * that our flushing had better include the xact's update of pg_clog.  So
	 * we wait till he's out of his commit critical section before proceeding.
	 * See notes in RecordTransactionCommit().
	 *
	 * Because we've already released the insertion slots, this test is a bit
	 * fuzzy: it is possible that we will wait for xacts we didn't really need
	 * to wait for.  But the delay should be short and it seems better to make
	 * checkpoint take a bit longer than to hold off insertions longer than
	 * necessary.
	 * (In fact, the whole reason we have this issue is that xact.c does
	 * commit record XLOG insertion and clog update as two separate steps
	 * protected by different locks, but again that seems best on grounds of
	 * minimizing lock contention.)
	 *
	 * A transaction that has not yet set delayChkpt when we look cannot be at
	 * risk, since he's not inserted his commit record yet; and one that's
	 * already cleared it is not at risk either, since he's done fixing clog
	 * and we will correctly flush the update below.  So we cannot miss any
	 * xacts we need to wait for.
	 */
	vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
	if (nvxids > 0)
	{
		do
		{
			pg_usleep(10000L);	/* wait for 10 msec */
		} while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
	}
	pfree(vxids);

	/*
	 * Get the other info we need for the checkpoint record.
	 */
	LWLockAcquire(XidGenLock, LW_SHARED);
	checkPoint.nextXid = ShmemVariableCache->nextXid;
	checkPoint.oldestXid = ShmemVariableCache->oldestXid;
	checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
	LWLockRelease(XidGenLock);

	/* Increase XID epoch if we've wrapped around since last checkpoint */
	checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
	if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
		checkPoint.nextXidEpoch++;

	LWLockAcquire(OidGenLock, LW_SHARED);
	checkPoint.nextOid = ShmemVariableCache->nextOid;
	if (!shutdown)
		checkPoint.nextOid += ShmemVariableCache->oidCount;
	LWLockRelease(OidGenLock);

	MultiXactGetCheckptMulti(shutdown,
							 &checkPoint.nextMulti,
							 &checkPoint.nextMultiOffset,
							 &checkPoint.oldestMulti,
							 &checkPoint.oldestMultiDB);

	/*
	 * Having constructed the checkpoint record, ensure all shmem disk buffers
	 * and commit-log buffers are flushed to disk.
	 *
	 * This I/O could fail for various reasons.  If so, we will fail to
	 * complete the checkpoint, but there is no reason to force a system
	 * panic. Accordingly, exit critical section while doing it.
	 */
	END_CRIT_SECTION();

	CheckPointGuts(checkPoint.redo, flags);

	/*
	 * Take a snapshot of running transactions and write this to WAL. This
	 * allows us to reconstruct the state of running transactions during
	 * archive recovery, if required. Skip, if this info disabled.
	 *
	 * If we are shutting down, or Startup process is completing crash
	 * recovery we don't need to write running xact data.
	 */
	if (!shutdown && XLogStandbyInfoActive())
		LogStandbySnapshot();

	START_CRIT_SECTION();

	/*
	 * Now insert the checkpoint record into XLOG.
	 */
	rdata.data = (char *) (&checkPoint);
	rdata.len = sizeof(checkPoint);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;

	recptr = XLogInsert(RM_XLOG_ID,
						shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
						XLOG_CHECKPOINT_ONLINE,
						&rdata);

	XLogFlush(recptr);

	/*
	 * We mustn't write any new WAL after a shutdown checkpoint, or it will be
	 * overwritten at next startup.  No-one should even try, this just allows
	 * sanity-checking.  In the case of an end-of-recovery checkpoint, we want
	 * to just temporarily disable writing until the system has exited
	 * recovery.
	 */
	if (shutdown)
	{
		if (flags & CHECKPOINT_END_OF_RECOVERY)
			LocalXLogInsertAllowed = -1;		/* return to "check" state */
		else
			LocalXLogInsertAllowed = 0; /* never again write WAL */
	}

	/*
	 * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
	 * = end of actual checkpoint record.
	 */
	if (shutdown && checkPoint.redo != ProcLastRecPtr)
		ereport(PANIC,
				(errmsg("concurrent transaction log activity while database system is shutting down")));

	/*
	 * Select point at which we can truncate the log, which we base on the
	 * prior checkpoint's earliest info.
	 */
	XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

	/*
	 * Update the control file.
	 */
	LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
	if (shutdown)
		ControlFile->state = DB_SHUTDOWNED;
	ControlFile->prevCheckPoint = ControlFile->checkPoint;
	ControlFile->checkPoint = ProcLastRecPtr;
	ControlFile->checkPointCopy = checkPoint;
	ControlFile->time = (pg_time_t) time(NULL);
	/* crash recovery should always recover to the end of WAL */
	ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
	ControlFile->minRecoveryPointTLI = 0;

	/*
	 * Persist unloggedLSN value. It's reset on crash recovery, so this goes
	 * unused on non-shutdown checkpoints, but seems useful to store it always
	 * for debugging purposes.
	 */
	SpinLockAcquire(&XLogCtl->ulsn_lck);
	ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
	SpinLockRelease(&XLogCtl->ulsn_lck);

	UpdateControlFile();
	LWLockRelease(ControlFileLock);

	/* Update shared-memory copy of checkpoint XID/epoch */
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile XLogCtlData *xlogctl = XLogCtl;

		SpinLockAcquire(&xlogctl->info_lck);
		xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
		xlogctl->ckptXid = checkPoint.nextXid;
		SpinLockRelease(&xlogctl->info_lck);
	}

	/*
	 * We are now done with critical updates; no need for system panic if we
	 * have trouble while fooling with old log segments.
	 */
	END_CRIT_SECTION();

	/*
	 * Let smgr do post-checkpoint cleanup (eg, deleting old files).
	 */
	smgrpostckpt();

	/*
	 * Delete old log files (those no longer needed even for previous
	 * checkpoint or the standbys in XLOG streaming).
	 */
	if (_logSegNo)
	{
		KeepLogSeg(recptr, &_logSegNo);
		_logSegNo--;
		RemoveOldXlogFiles(_logSegNo, recptr);
	}

	/*
	 * Make more log segments if needed.  (Do this after recycling old log
	 * segments, since that may supply some of the needed files.)
	 */
	if (!shutdown)
		PreallocXlogFiles(recptr);

	/*
	 * Truncate pg_subtrans if possible.  We can throw away all data before
	 * the oldest XMIN of any running transaction.	No future transaction will
	 * attempt to reference any pg_subtrans entry older than that (see Asserts
	 * in subtrans.c).	During recovery, though, we mustn't do this because
	 * StartupSUBTRANS hasn't been called yet.
	 */
	if (!RecoveryInProgress())
		TruncateSUBTRANS(GetOldestXmin(true, false));

	/* Real work is done, but log and update stats before releasing lock. */
	LogCheckpointEnd(false);

	TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
									 NBuffers,
									 CheckpointStats.ckpt_segs_added,
									 CheckpointStats.ckpt_segs_removed,
									 CheckpointStats.ckpt_segs_recycled);

	LWLockRelease(CheckpointLock);
}