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

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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数据库远程运维 PostgreSQL实战之启动恢复读取checkpoint记录失败的条件详解