标签

PostgreSQL , checkpoint , standby , CreateRestartPoint

背景

当数据库异常关闭时，数据库关闭时来不及或根本没有机会创建一个一致性的检查点，所以需要从上一个一致性检查点开始恢复。

实际上是数据库启动时检查控制文件中的数据库状态，如果状态不是shutdown的，那么说明数据库是异常关闭的（当然我们说，除了recovery状态），总之就需要从检查点开始恢复。

src/include/catalog/pg_control.h

/*   * System status indicator.  Note this is stored in pg_control; if you change   * it, you must bump PG_CONTROL_VERSION   */  typedef enum DBState  {          DB_STARTUP = 0,          DB_SHUTDOWNED,          DB_SHUTDOWNED_IN_RECOVERY,          DB_SHUTDOWNING,          DB_IN_CRASH_RECOVERY,          DB_IN_ARCHIVE_RECOVERY,          DB_IN_PRODUCTION  } DBState;

那么问题来了，对于standby，一直是出于recovery状态的，即使停库也可能是这个状态(实际上是shutdown_in_recovery)，所以STANDBY停库再起来，理论上来说又需要重上一个检查点开始恢复WAL，即使这个数据库已经恢复到了最新的WAL。

为了提高性能，缩短recovery时间，pg在standby上面也支持了checkpoint，叫做CreateRestartPoint。

CreateRestartPoint@src/backend/access/transam/xlog.c

/*   * 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)  {

当standby关闭，重启时，从restartpoint开始APPLY WAL，而不是上游节点（主节点）在WAL日志中写的检查点开始。

停库时，备库写检查点

/*   * This must be called ONCE during postmaster or standalone-backend shutdown   */  void  ShutdownXLOG(int code, Datum arg)  {          /* Don't be chatty in standalone mode */          ereport(IsPostmasterEnvironment ? LOG : NOTICE,                          (errmsg("shutting down")));            /*           * Signal walsenders to move to stopping state.           */          WalSndInitStopping();            /*           * Wait for WAL senders to be in stopping state.  This prevents commands           * from writing new WAL.           */          WalSndWaitStopping();            if (RecoveryInProgress())   // 备库写检查点                  CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);          else          {                  /*                   * If archiving is enabled, rotate the last XLOG file so that all the                   * remaining records are archived (postmaster wakes up the archiver                   * process one more time at the end of shutdown). The checkpoint                   * record will go to the next XLOG file and won't be archived (yet).                   */                  if (XLogArchivingActive() && XLogArchiveCommandSet())                          RequestXLogSwitch(false);                    CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);          }          ShutdownCLOG();          ShutdownCommitTs();          ShutdownSUBTRANS();          ShutdownMultiXact();  }

CreateRestartPoint    ...          LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);          if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&                  ControlFile->checkPointCopy.redo < lastCheckPoint.redo)          {                  ControlFile->checkPoint = lastCheckPointRecPtr;                  ControlFile->checkPointCopy = lastCheckPoint;                  ControlFile->time = (pg_time_t) time(NULL);                    /*                   * Ensure minRecoveryPoint is past the checkpoint record.  Normally,                   * this will have happened already while writing out dirty buffers,                   * but not necessarily - e.g. because no buffers were dirtied.  We do                   * this because a non-exclusive base backup uses minRecoveryPoint to                   * determine which WAL files must be included in the backup, and the                   * file (or files) containing the checkpoint record must be included,                   * at a minimum. Note that for an ordinary restart of recovery there's                   * no value in having the minimum recovery point any earlier than this                   * anyway, because redo will begin just after the checkpoint record.                   */                  if (ControlFile->minRecoveryPoint < lastCheckPointEndPtr)                  {                          ControlFile->minRecoveryPoint = lastCheckPointEndPtr;                          ControlFile->minRecoveryPointTLI = lastCheckPoint.ThisTimeLineID;                            /* update local copy */                          minRecoveryPoint = ControlFile->minRecoveryPoint;                          minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;                  }                  if (flags & CHECKPOINT_IS_SHUTDOWN)                          ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;  // 备库正常关闭时，状态改写为DB_SHUTDOWNED_IN_RECOVERY                  UpdateControlFile();          }

小结

PostgreSQL备库也可以写检查点，目的是避免每次重启备库都需要从上一个检查点（由主库产生，在WAL中回放出来的）APPLY后面所有的WAL。

从而forward 备库停库后重启时WAL的位点。

参考

CreateRestartPoint@src/backend/access/transam/xlog.c