接前文 初步学习pg_control文件之九 看下面这个
XLogRecPtr checkPoint; /* last check point record ptr */
看看这个pointer究竟保留了什么
初始化的时候：
/* * This func must be called ONCE on system install. It creates pg_control * and the initial XLOG segment. */ void BootStrapXLOG(void) { … /* * Set up information for the initial checkpoint record * * The initial checkpoint record is written to the beginning of the WAL * segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not * used, so that we can use 0/0 to mean "before any valid WAL segment". */ checkPoint.redo.xlogid = 0; checkPoint.redo.xrecoff = XLogSegSize + SizeOfXLogLongPHD; ... /* Now create pg_control */ memset(ControlFile, 0, sizeof(ControlFileData)); /* Initialize pg_control status fields */ ControlFile->system_identifier = sysidentifier; ControlFile->state = DB_SHUTDOWNED; ControlFile->time = checkPoint.time; ControlFile->checkPoint = checkPoint.redo; ControlFile->checkPointCopy = checkPoint; … WriteControlFile(); … }
运行的时候：
/* * ProcLastRecPtr points to the start of the last XLOG record inserted by the * current backend. It is updated for all inserts. XactLastRecEnd points to * end+1 of the last record, and is reset when we end a top-level transaction, * or start a new one; so it can be used to tell if the current transaction has * created any XLOG records. */ static XLogRecPtr ProcLastRecPtr = {0, 0}; … /* * Insert an XLOG record having the specified RMID and info bytes, * with the body of the record being the data chunk(s) described by * the rdata chain (see xlog.h for notes about rdata). * * Returns XLOG pointer to end of record (beginning of next record). * This can be used as LSN for data pages affected by the logged action. * (LSN is the XLOG point up to which the XLOG must be flushed to disk * before the data page can be written out. This implements the basic * WAL rule "write the log before the data".) * * NB: this routine feels free to scribble on the XLogRecData structs, * though not on the data they reference. This is OK since the XLogRecData * structs are always just temporaries in the calling code. */ XLogRecPtr XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata) { … /* * In bootstrap mode, we don't actually log anything but XLOG resources; * return a phony record pointer. */ if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID) { RecPtr.xlogid = 0; RecPtr.xrecoff = SizeOfXLogLongPHD; /* start of 1st chkpt record */ return RecPtr; } … /* Record begin of record in appropriate places */ ProcLastRecPtr = RecPtr; Insert->PrevRecord = RecPtr; … }
关闭的时候：
/* * Perform a checkpoint --- either during shutdown, or on-the-fly * * flags is a bitwise OR of the following: * CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown. * CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery. * CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP, * ignoring checkpoint_completion_target parameter. * CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occured * since the last one (implied by CHECKPOINT_IS_SHUTDOWN or * CHECKPOINT_END_OF_RECOVERY). * * Note: flags contains other bits, of interest here only for logging purposes. * In particular note that this routine is synchronous and does not pay * attention to CHECKPOINT_WAIT. */ void CreateCheckPoint(int flags) { … /* * 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 */ MemSet(&ControlFile->minRecoveryPoint, 0, sizeof(XLogRecPtr)); UpdateControlFile(); LWLockRelease(ControlFileLock); … }
RecoveryMode或者Stand by 的时候：
/* * Establish a restartpoint if possible. * * This is similar to CreateCheckPoint, but is used during WAL recovery * to establish a point from which recovery can roll forward without * replaying the entire recovery log. * * Returns true if a new restartpoint was established. We can only establish * a restartpoint if we have replayed a safe checkpoint record since last * restartpoint. */ bool CreateRestartPoint(int flags) { XLogRecPtr lastCheckPointRecPtr; CheckPoint lastCheckPoint; uint32 _logId; uint32 _logSeg; TimestampTz xtime; /* use volatile pointer to prevent code rearrangement */ volatile XLogCtlData *xlogctl = XLogCtl; /* * Acquire CheckpointLock to ensure only one restartpoint or checkpoint * happens at a time. */ LWLockAcquire(CheckpointLock, LW_EXCLUSIVE); /* Get a local copy of the last safe checkpoint record. */ SpinLockAcquire(&xlogctl->info_lck); lastCheckPointRecPtr = xlogctl->lastCheckPointRecPtr; memcpy(&lastCheckPoint, &XLogCtl->lastCheckPoint, sizeof(CheckPoint)); SpinLockRelease(&xlogctl->info_lck); … /* * Update pg_control, using current time. Check that it still shows * IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing; * this is a quick hack to make sure nothing really bad happens if somehow * we get here after the end-of-recovery checkpoint. */ LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY && XLByteLT(ControlFile->checkPointCopy.redo, lastCheckPoint.redo)) { ControlFile->prevCheckPoint = ControlFile->checkPoint; ControlFile->checkPoint = lastCheckPointRecPtr; ControlFile->checkPointCopy = lastCheckPoint; ControlFile->time = (pg_time_t) time(NULL); if (flags & CHECKPOINT_IS_SHUTDOWN) ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY; UpdateControlFile(); } LWLockRelease(ControlFileLock); … }
启动的时候：
/* * This must be called ONCE during postmaster or standalone-backend startup */ void StartupXLOG(void) { … /* * 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(); if (ControlFile->state < DB_SHUTDOWNED || ControlFile->state > DB_IN_PRODUCTION || !XRecOffIsValid(ControlFile->checkPoint.xrecoff)) ereport(FATAL, (errmsg("control file contains invalid data"))); … if (read_backup_label(&checkPointLoc, &backupEndRequired)) { … } else { /* * Get the last valid checkpoint record. If the latest one according * to pg_control is broken, try the next-to-last one. */ checkPointLoc = ControlFile->checkPoint; RedoStartLSN = ControlFile->checkPointCopy.redo; record = ReadCheckpointRecord(checkPointLoc, 1); … } … /* REDO */ if (InRecovery) { … ControlFile->prevCheckPoint = ControlFile->checkPoint; ControlFile->checkPoint = checkPointLoc; ControlFile->checkPointCopy = checkPoint; … } … }