clog.c

postgresql8.3.4源码,开源数据库

 */
void
StartupCLOG(void)
{
	TransactionId xid = ShmemVariableCache->nextXid;
	int			pageno = TransactionIdToPage(xid);

	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

	/*
	 * Initialize our idea of the latest page number.
	 */
	ClogCtl->shared->latest_page_number = pageno;

	/*
	 * Zero out the remainder of the current clog page.  Under normal
	 * circumstances it should be zeroes already, but it seems at least
	 * theoretically possible that XLOG replay will have settled on a nextXID
	 * value that is less than the last XID actually used and marked by the
	 * previous database lifecycle (since subtransaction commit writes clog
	 * but makes no WAL entry).  Let's just be safe. (We need not worry about
	 * pages beyond the current one, since those will be zeroed when first
	 * used.  For the same reason, there is no need to do anything when
	 * nextXid is exactly at a page boundary; and it's likely that the
	 * "current" page doesn't exist yet in that case.)
	 */
	if (TransactionIdToPgIndex(xid) != 0)
	{
		int			byteno = TransactionIdToByte(xid);
		int			bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
		int			slotno;
		char	   *byteptr;

		slotno = SimpleLruReadPage(ClogCtl, pageno, false, xid);
		byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;

		/* Zero so-far-unused positions in the current byte */
		*byteptr &= (1 << bshift) - 1;
		/* Zero the rest of the page */
		MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);

		ClogCtl->shared->page_dirty[slotno] = true;
	}

	LWLockRelease(CLogControlLock);
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownCLOG(void)
{
	/* Flush dirty CLOG pages to disk */
	SimpleLruFlush(ClogCtl, false);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 */
void
CheckPointCLOG(void)
{
	/* Flush dirty CLOG pages to disk */
	SimpleLruFlush(ClogCtl, true);
}


/*
 * Make sure that CLOG has room for a newly-allocated XID.
 *
 * NB: this is called while holding XidGenLock.  We want it to be very fast
 * most of the time; even when it's not so fast, no actual I/O need happen
 * unless we're forced to write out a dirty clog or xlog page to make room
 * in shared memory.
 */
void
ExtendCLOG(TransactionId newestXact)
{
	int			pageno;

	/*
	 * No work except at first XID of a page.  But beware: just after
	 * wraparound, the first XID of page zero is FirstNormalTransactionId.
	 */
	if (TransactionIdToPgIndex(newestXact) != 0 &&
		!TransactionIdEquals(newestXact, FirstNormalTransactionId))
		return;

	pageno = TransactionIdToPage(newestXact);

	LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

	/* Zero the page and make an XLOG entry about it */
	ZeroCLOGPage(pageno, true);

	LWLockRelease(CLogControlLock);
}


/*
 * Remove all CLOG segments before the one holding the passed transaction ID
 *
 * Before removing any CLOG data, we must flush XLOG to disk, to ensure
 * that any recently-emitted HEAP_FREEZE records have reached disk; otherwise
 * a crash and restart might leave us with some unfrozen tuples referencing
 * removed CLOG data.  We choose to emit a special TRUNCATE XLOG record too.
 * Replaying the deletion from XLOG is not critical, since the files could
 * just as well be removed later, but doing so prevents a long-running hot
 * standby server from acquiring an unreasonably bloated CLOG directory.
 *
 * Since CLOG segments hold a large number of transactions, the opportunity to
 * actually remove a segment is fairly rare, and so it seems best not to do
 * the XLOG flush unless we have confirmed that there is a removable segment.
 */
void
TruncateCLOG(TransactionId oldestXact)
{
	int			cutoffPage;

	/*
	 * The cutoff point is the start of the segment containing oldestXact. We
	 * pass the *page* containing oldestXact to SimpleLruTruncate.
	 */
	cutoffPage = TransactionIdToPage(oldestXact);

	/* Check to see if there's any files that could be removed */
	if (!SlruScanDirectory(ClogCtl, cutoffPage, false))
		return;					/* nothing to remove */

	/* Write XLOG record and flush XLOG to disk */
	WriteTruncateXlogRec(cutoffPage);

	/* Now we can remove the old CLOG segment(s) */
	SimpleLruTruncate(ClogCtl, cutoffPage);
}


/*
 * Decide which of two CLOG page numbers is "older" for truncation purposes.
 *
 * We need to use comparison of TransactionIds here in order to do the right
 * thing with wraparound XID arithmetic.  However, if we are asked about
 * page number zero, we don't want to hand InvalidTransactionId to
 * TransactionIdPrecedes: it'll get weird about permanent xact IDs.  So,
 * offset both xids by FirstNormalTransactionId to avoid that.
 */
static bool
CLOGPagePrecedes(int page1, int page2)
{
	TransactionId xid1;
	TransactionId xid2;

	xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
	xid1 += FirstNormalTransactionId;
	xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
	xid2 += FirstNormalTransactionId;

	return TransactionIdPrecedes(xid1, xid2);
}


/*
 * Write a ZEROPAGE xlog record
 */
static void
WriteZeroPageXlogRec(int pageno)
{
	XLogRecData rdata;

	rdata.data = (char *) (&pageno);
	rdata.len = sizeof(int);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;
	(void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE, &rdata);
}

/*
 * Write a TRUNCATE xlog record
 *
 * We must flush the xlog record to disk before returning --- see notes
 * in TruncateCLOG().
 */
static void
WriteTruncateXlogRec(int pageno)
{
	XLogRecData rdata;
	XLogRecPtr	recptr;

	rdata.data = (char *) (&pageno);
	rdata.len = sizeof(int);
	rdata.buffer = InvalidBuffer;
	rdata.next = NULL;
	recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE, &rdata);
	XLogFlush(recptr);
}

/*
 * CLOG resource manager's routines
 */
void
clog_redo(XLogRecPtr lsn, XLogRecord *record)
{
	uint8		info = record->xl_info & ~XLR_INFO_MASK;

	if (info == CLOG_ZEROPAGE)
	{
		int			pageno;
		int			slotno;

		memcpy(&pageno, XLogRecGetData(record), sizeof(int));

		LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);

		slotno = ZeroCLOGPage(pageno, false);
		SimpleLruWritePage(ClogCtl, slotno, NULL);
		Assert(!ClogCtl->shared->page_dirty[slotno]);

		LWLockRelease(CLogControlLock);
	}
	else if (info == CLOG_TRUNCATE)
	{
		int			pageno;

		memcpy(&pageno, XLogRecGetData(record), sizeof(int));

		/*
		 * During XLOG replay, latest_page_number isn't set up yet; insert a
		 * suitable value to bypass the sanity test in SimpleLruTruncate.
		 */
		ClogCtl->shared->latest_page_number = pageno;

		SimpleLruTruncate(ClogCtl, pageno);
	}
	else
		elog(PANIC, "clog_redo: unknown op code %u", info);
}

void
clog_desc(StringInfo buf, uint8 xl_info, char *rec)
{
	uint8		info = xl_info & ~XLR_INFO_MASK;

	if (info == CLOG_ZEROPAGE)
	{
		int			pageno;

		memcpy(&pageno, rec, sizeof(int));
		appendStringInfo(buf, "zeropage: %d", pageno);
	}
	else if (info == CLOG_TRUNCATE)
	{
		int			pageno;

		memcpy(&pageno, rec, sizeof(int));
		appendStringInfo(buf, "truncate before: %d", pageno);
	}
	else
		appendStringInfo(buf, "UNKNOWN");
}