接前问，初步学习pg_control文件之十一，再来看下面这个
XLogRecPtr minRecoveryPoint;
看其注释：
* minRecoveryPoint is updated to the latest replayed LSN whenever we * flush a data change during archive recovery. That guards against * starting archive recovery, aborting it, and restarting with an earlier * stop location. If we've already flushed data changes from WAL record X * to disk, we mustn't start up until we reach X again. Zero when not * doing archive recovery.
看来，是为了防止出现重复执行重做日志...它强调的是在 archive recovery的情况下。
启动时，如果处于Recovery状态，则要进行设置：
/* * This must be called ONCE during postmaster or standalone-backend startup */ void StartupXLOG(void) { … CheckPoint checkPoint; … /* * 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"))); … /* REDO */ if (InRecovery) { … if (InArchiveRecovery) { /* initialize minRecoveryPoint if not set yet */ if (XLByteLT(ControlFile->minRecoveryPoint, checkPoint.redo)) ControlFile->minRecoveryPoint = checkPoint.redo; } … } … }
在xlogredo函数中得到处理：
/* * XLOG resource manager's routines * * Definitions of info values are in include/catalog/pg_control.h, though * not all record types are related to control file updates. */ void xlog_redo(XLogRecPtr lsn, XLogRecord *record) { … if (info == XLOG_NEXTOID) { … } … else if (info == XLOG_BACKUP_END) { … if (XLByteEQ(ControlFile->backupStartPoint, startpoint)) { /* * We have reached the end of base backup, the point where * pg_stop_backup() was done. The data on disk is now consistent. * Reset backupStartPoint, and update minRecoveryPoint to make * sure we don't allow starting up at an earlier point even if * recovery is stopped and restarted soon after this. */ … LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); if (XLByteLT(ControlFile->minRecoveryPoint, lsn)) ControlFile->minRecoveryPoint = lsn; MemSet(&ControlFile->backupStartPoint, 0, sizeof(XLogRecPtr)); UpdateControlFile(); LWLockRelease(ControlFileLock); } } else if (info == XLOG_PARAMETER_CHANGE) { … LWLockAcquire(ControlFileLock, LW_EXCLUSIVE); ControlFile->MaxConnections = xlrec.MaxConnections; ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts; ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact; ControlFile->wal_level = xlrec.wal_level; /* * Update minRecoveryPoint to ensure that if recovery is aborted, we * recover back up to this point before allowing hot standby again. * This is particularly important if wal_level was set to 'archive' * before, and is now 'hot_standby', to ensure you don't run queries * against the WAL preceding the wal_level change. Same applies to * decreasing max_* settings. */ minRecoveryPoint = ControlFile->minRecoveryPoint; if ((minRecoveryPoint.xlogid != 0 || minRecoveryPoint.xrecoff != 0) && XLByteLT(minRecoveryPoint, lsn)) { ControlFile->minRecoveryPoint = lsn; } UpdateControlFile(); LWLockRelease(ControlFileLock); … } }
再就是checkpoint发生的时候，也做了处理：
CreateCheckPoint -->XLogFlush--> UpdateMinRecoveryPoint
/* * 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) { … recptr = XLogInsert(RM_XLOG_ID, shutdown ? XLOG_CHECKPOINT_SHUTDOWN : XLOG_CHECKPOINT_ONLINE, &rdata); XLogFlush(recptr); … }
/* * Ensure that all XLOG data through the given position is flushed to disk. * * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not * already held, and we try to avoid acquiring it if possible. */ void XLogFlush(XLogRecPtr record) { XLogRecPtr WriteRqstPtr; XLogwrtRqst WriteRqst; /* * During REDO, we are reading not writing WAL. Therefore, instead of * trying to flush the WAL, we should update minRecoveryPoint instead. We * test XLogInsertAllowed(), not InRecovery, because we need the bgwriter * to act this way too, and because when the bgwriter tries to write the * end-of-recovery checkpoint, it should indeed flush. */ if (!XLogInsertAllowed()) { UpdateMinRecoveryPoint(record, false); return; } /* Quick exit if already known flushed */ if (XLByteLE(record, LogwrtResult.Flush)) return; … }
/* * Advance minRecoveryPoint in control file. * If we crash during recovery, we must reach this point again before the * database is consistent. * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint * is only updated if it's not already greater than or equal to 'lsn'. */ static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force) { … /* * An invalid minRecoveryPoint means that we need to recover all the WAL, * i.e., we're doing crash recovery. We never modify the control file's * value in that case, so we can short-circuit future checks here too. */ if (minRecoveryPoint.xlogid == 0 && minRecoveryPoint.xrecoff == 0) updateMinRecoveryPoint = false; else if (force || XLByteLT(minRecoveryPoint, lsn)) { /* use volatile pointer to prevent code rearrangement */ volatile XLogCtlData *xlogctl = XLogCtl; XLogRecPtr newMinRecoveryPoint; /* * To avoid having to update the control file too often, we update it * all the way to the last record being replayed, even though 'lsn' * would suffice for correctness. This also allows the 'force' case * to not need a valid 'lsn' value. * * Another important reason for doing it this way is that the passed * 'lsn' value could be bogus, i.e., past the end of available WAL, if * the caller got it from a corrupted heap page. Accepting such a * value as the min recovery point would prevent us from coming up at * all. Instead, we just log a warning and continue with recovery. * (See also the comments about corrupt LSNs in XLogFlush.) */ SpinLockAcquire(&xlogctl->info_lck); newMinRecoveryPoint = xlogctl->replayEndRecPtr; SpinLockRelease(&xlogctl->info_lck); if (!force && XLByteLT(newMinRecoveryPoint, lsn)) elog(WARNING, "xlog min recovery request %X/%X is past current point %X/%X", lsn.xlogid, lsn.xrecoff, newMinRecoveryPoint.xlogid, newMinRecoveryPoint.xrecoff); /* update control file */ if (XLByteLT(ControlFile->minRecoveryPoint, newMinRecoveryPoint)) { ControlFile->minRecoveryPoint = newMinRecoveryPoint; UpdateControlFile(); minRecoveryPoint = newMinRecoveryPoint; ereport(DEBUG2, (errmsg("updated min recovery point to %X/%X", minRecoveryPoint.xlogid, minRecoveryPoint.xrecoff))); } } LWLockRelease(ControlFileLock); }
