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

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