PG数据库内核分析学习笔记_XLOG日志恢复策略

PG数据库内核分析学习笔记_XLOG日志恢复策略

在PostgreSQL中，系统在崩溃后重新启动时会调用StartupXlog入口函数.

// xlog.c
/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
   
   
	XLogCtlInsert *Insert;
	CheckPoint	checkPoint;
	bool		wasShutdown;
	bool		reachedStopPoint = false;
	bool		haveBackupLabel = false;
	bool		haveTblspcMap = false;
	XLogRecPtr	RecPtr,
				checkPointLoc,
				EndOfLog;
	TimeLineID	EndOfLogTLI;
	TimeLineID	PrevTimeLineID;
	XLogRecord *record;
	TransactionId oldestActiveXID;
	bool		backupEndRequired = false;
	bool		backupFromStandby = false;
	DBState		dbstate_at_startup;
	XLogReaderState *xlogreader;
	XLogPageReadPrivate private;
	bool		fast_promoted = false;
	struct stat st;

该函数首先会扫描全 局信息控制文件（global/pg_control）读取系统的控制信息.

/*
	 * Read control file and check XLOG status looks valid.
	 *
	 * Note: in most control paths, *ControlFile is already valid and we need
	 * not do ReadControlFile() here, but might as well do it to be sure.
	 */
	ReadControlFile();

然后扫描XLOG 0志目录的结构检测其 是否完整.

/*
	 * Verify that pg_wal and pg_wal/archive_status exist.  In cases where
	 * someone has performed a copy for PITR, these directories may have been
	 * excluded and need to be re-created.
	 */
	ValidateXLOGDirectoryStructure();

进而读取到最新的日志检査点记录.

/*
		 * When a backup_label file is present, we want to roll forward from
		 * the checkpoint it identifies, rather than using pg_control.
		 */
		record = ReadCheckpointRecord(xlogreader, checkPointLoc, 0, true);
		if (record != NULL)

接下来根据日志记录序列的偏序关系检测到系统是 否处于非正常状态下，若系统处于非正常状态，则触发恢复机制进行恢复。恢复完成后重新建立检 査点并初始化XlogCtl控制信息，然后启动事务提交日志以及相关辅助日志模块。

日志中岀现以下三种情况需要进行恢复操作

1） 日志文件中扫描到backup_label文件。

if