看这个：
* backupStartPoint is the redo pointer of the backup start checkpoint, if * we are recovering from an online backup and haven't reached the end of * backup yet. It is reset to zero when the end of backup is reached, and * we mustn't start up before that. A boolean would suffice otherwise, but * we use the redo pointer as a cross-check when we see an end-of-backup * record, to make sure the end-of-backup record corresponds the base * backup we're recovering from. */ ... XLogRecPtr backupStartPoint;
是说如果处在online backup中时会使用它。只有下面一个地方进行了赋值：
/* * This must be called ONCE during postmaster or standalone-backend startup */ void StartupXLOG(void) { … /* REDO */ if (InRecovery) { … /* * Set backupStartPoint if we're starting recovery from a base backup. * However, if there was no recovery.conf, and the backup was taken * with pg_start_backup(), we don't know if the server crashed before * the backup was finished and we're doing crash recovery on the * original server, or if we're restoring from the base backup. We * have to assume we're doing crash recovery in that case, or the * database would refuse to start up after a crash. */ if ((InArchiveRecovery && haveBackupLabel) || backupEndRequired) ControlFile->backupStartPoint = checkPoint.redo; ControlFile->time = (pg_time_t) time(NULL); /* No need to hold ControlFileLock yet, we aren't up far enough */ UpdateControlFile(); … } … }