PostgreSQL实战之启动恢复读取checkpoint记录失败的条件详解

    1、首先读取ControlFile->checkPoint指向的checkpoint

    2、如果读取失败,slave直接abort退出,master再次读取ControlFile->prevCheckPoint指向的checkpoint

    
    StartupXLOG->
     |--checkPointLoc = ControlFile->checkPoint;
     |--record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, true):
     |-- if (record != NULL){
       ...
      }else if (StandbyMode){
       ereport(PANIC,(errmsg("could not locate a valid checkpoint record")));
      }else{
       checkPointLoc = ControlFile->prevCheckPoint;
       record = ReadCheckpointRecord(xlogreader, checkPointLoc, 2, true);
       if (record != NULL){
        InRecovery = true;//标记下面进入recovery
       }else{
        ereport(PANIC,(errmsg("could not locate a valid checkpoint record")));
       }
      }

    一、那么什么条件下读取的checkpoint记录record==NULL?

    1、ControlFile->checkPoint % XLOG_BLCKSZ < SizeOfXLogShortPHD
    2、ReadRecord(xlogreader, ControlFile->checkPoint, LOG, true)返回NULL
    3、ReadRecord读到的record!=NULL && record->xl_rmid != RM_XLOG_ID
    4、ReadRecord读到的record!=NULL && info != XLOG_CHECKPOINT_SHUTDOWN && info != XLOG_CHECKPOINT_ONLINE
    5、ReadRecord读到的record!=NULL && record->xl_tot_len != SizeOfXLogRecord + SizeOfXLogRecordDataHeaderShort + sizeof(CheckPoint)

    二、ReadRecord函数返回NULL的条件

    
    ReadRecord(xlogreader, ControlFile->checkPoint, LOG, true)
     |--record = XLogReadRecord(xlogreader, ControlFile->checkPoint, &errormsg);
     |-- 2.1 record==NULL && !StandbyMode
     |-- 2.2 record!=NULL && !tliInHistory(xlogreader->latestPageTLI, expectedTLEs)
     /*-----
     note:只要读取了一页xlog,就会赋值为该页第一个记录的时间线
     XLogReaderValidatePageHeader
      -->xlogreader->latestPageTLI=hdr->xlp_tli;
     ------*/

    三、XlogReadRecord读取checkpoint返回NULL的条件?

    XLogReadRecord(xlogreader, ControlFile->checkPoint, &errormsg)
        targetPagePtr = ControlFile->checkPoint – (ControlFile->checkPoint % XLOG_BLCKSZ);
        targetRecOff = ControlFile->checkPoint % XLOG_BLCKSZ;
        readOff = ReadPageInternal(state,targetPagePtr, Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ));
        pageHeaderSize = XLogPageHeaderSize((XLogPageHeader) state->readBuf);
        record = (XLogRecord *) (state->readBuf + RecPtr % XLOG_BLCKSZ);
        total_len = record->xl_tot_len;
        ————-
        1、readOff < 0
        2、0< targetRecOff < pageHeaderSize
        3、(((XLogPageHeader) state->readBuf)->xlp_info & XLP_FIRST_IS_CONTRECORD) && targetRecOff == pageHeaderSize
           page头有跨页的record并且checkpoint定位的偏移正好在页头尾部
        4、targetRecOff <= XLOG_BLCKSZ – SizeOfXLogRecord &&
           !ValidXLogRecordHeader(state, ControlFile->checkPoint, state->ReadRecPtr, record,randAccess)
           —(record->xl_tot_len < SizeOfXLogRecord || record->xl_rmid > RM_MAX_ID || record->xl_prev != state->ReadRecPtr)
        5、targetRecOff > XLOG_BLCKSZ – SizeOfXLogRecord && total_len < SizeOfXLogRecord
        6、total_len > state->readRecordBufSize && !allocate_recordbuf(state, total_len)
           一旦该记录损坏,total_len的长度非常大的话,就需要allocate_recordbuf扩展state->readbuf,可能因此分配失败abort
           记录的checksum需要等待全部读取完整记录后才校验
        ————-

    三、ReadPageInternal返回的readOff返回小于0的条件

    
    ReadPageInternal(state,targetPagePtr, Min(targetRecOff + SizeOfXLogRecord, XLOG_BLCKSZ))

        1、第一次read wal文件,readLen = state->read_page:读取第一页。readLen < 0

        2、readLen>0 && !XLogReaderValidatePageHeader(state, targetSegmentPtr, state->readBuf)
        —

        3、读取checkpoint所在页readLen = state->read_page: readLen < 0

        4、readLen > 0 && readLen <= SizeOfXLogShortPHD

        5、!XLogReaderValidatePageHeader(state, pageptr, (char *) hdr)

    四、XLogPageRead何时返回值<0 ?

    
    /*
     1、WaitForWALToBecomeAvailable open失败
     2、lseek 失败 && !StandbyMode
     3、read失败 && !StandbyMode
     4、校验page头失败 && !StandbyMode
     如果是StandbyMode,则会重新retry->WaitForWALToBecomeAvailable,切换日志源进行open
     */
     !WaitForWALToBecomeAvailable(targetPagePtr + reqLen,private->randAccess,1,targetRecPtr)//open
     |-- return -1
     readOff = targetPageOff;
     if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0){
      !StandbyMode:: return -1
     }
     if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ){
      !StandbyMode:: return -1
     }
     XLogReaderValidatePageHeader(xlogreader, targetPagePtr, readBuf)
     !StandbyMode:: return -1

    五、WaitForWALToBecomeAvailable何时返回false?

    –XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL
        1、先XLogFileReadAnyTLI open日志:
            1、遍历时间线列表里的每一个时间线,从最新的开始
            2、当读取checkpoint的时候,source是XLOG_FROM_ANY
            3、先找归档的日志进行open;如果open失败再找WAL日志进行open
            4、如果都没有open成功,则向前找时间线,open前一个时间线segno和文件号相同的文件进行open
            5、open成功后expectedTLEs被赋值为当前时间线列表的所有值
        2、如果open失败,则切换日志源:XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL -> XLOG_FROM_STREAM
        3、切换日志源后,XLOG_FROM_ARCHIVE | XLOG_FROM_PG_WAL 则:
           slave && promote :return false
           !StandbyMode:return false
        –XLOG_FROM_STREAM
        1、!WalRcvStreaming()即receiver进程挂了,切换日志源
        2、CheckForStandbyTrigger()切换日志源
        3、XLOG_FROM_STREAM->XLOG_FROM_ARCHIVE

    总结

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