初步学习pg_control文件之十一

接前文  初步学习pg_control文件之十，再看这个

XLogRecPtr    prevCheckPoint; /* previous check point record ptr */

发生了checkpoint的时候，肯定要处理的：

/*
 * 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, , sizeof(XLogRecPtr));
    UpdateControlFile();
    LWLockRelease(ControlFileLock);
    …
}                            

可是为何要保留这个东西呢，看下面这段代码就知道了：

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
    …
    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, );
        if (record != NULL)
        {
            ereport(DEBUG1,
                    (errmsg("checkpoint record is at %X/%X",
                            checkPointLoc.xlogid, checkPointLoc.xrecoff)));
        }
        else if (StandbyMode)
        {
            /*
             * The last valid checkpoint record required for a streaming
             * recovery exists in neither standby nor the primary.
             */
            ereport(PANIC,
                    (errmsg("could not locate a valid checkpoint record")));
        }
        else
        {
            checkPointLoc = ControlFile->prevCheckPoint;
            record = ReadCheckpointRecord(checkPointLoc, );
            if (record != NULL)
            {
                ereport(LOG,
                        (errmsg("using previous checkpoint record at %X/%X",
                              checkPointLoc.xlogid, checkPointLoc.xrecoff)));
                InRecovery = true;        /* force recovery even if SHUTDOWNED */
            }
            else
                ereport(PANIC,
                     (errmsg("could not locate a valid checkpoint record")));
        }
        …
    }
    …
    /* REDO */
    if (InRecovery)
    {
        …
        /*
         * Update pg_control to show that we are recovering and to show the
         * selected checkpoint as the place we are starting from. We also mark
         * pg_control with any minimum recovery stop point obtained from a
         * backup history file.
         */
        if (InArchiveRecovery)
            ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
        else
        {
            ereport(LOG,
                    (errmsg("database system was not properly shut down; "
                            automatic recovery in progress)));
            ControlFile->state = DB_IN_CRASH_RECOVERY;
        }
        ControlFile->prevCheckPoint = ControlFile->checkPoint;
        ControlFile->checkPoint = checkPointLoc;
        ControlFile->checkPointCopy = checkPoint;
        …
    }
    …
}                                        

就是说，如果拿不到最好的，就拿到上次的那个checkpoint1点：Get the last valid checkpoint record. If the latest one according  to pg_control is broken, try the next-to-last one.  

此外，我们可以看restartPoint时，也有此处理：

/*
 * 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)
{
    …
    /*
     * 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);
    …
}