PostgreSQL备机checkpoint

yzsDBA

发布于 2020-10-29 10:50:29

7720

发布于 2020-10-29 10:50:29

文章被收录于专栏：PostgreSQL研究与原理解析

数据库异常关闭时，数据库关闭时来不及或者没机会做checkpoint，则需要从上一个一致性检查的开始恢复。

PostgreSQL备机checkpoint是不能产生checkpoint WAL的，因为如果写这样类型的checkpoint的话，就会将接收的WAL打乱，那么日志将混乱，回放会出问题。

那么问题来了，备机支持checkpoint吗？他的checkpoint怎么做的？

PostgreSQL为了缩短恢复时间，备机上也支持checkpoint，即CreateRestartPoint。但是其pg_control文件的checkpoint记录的位点是从主机传过来WAL里面的checkpoint记录位置。

调用流程：

StartupXLOG
  do{
    ...
    RmgrTable[record->xl_rmid].rm_redo(xlogreader);
    |--  RecoveryRestartPoint(&checkPoint);
      |--  lastCheckPointRecPtr = XLogCtl->lastCheckPointRecPtr;
      |--  ControlFile->checkPoint = lastCheckPointRecPtr;
    ...
    record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
    |--  record = XLogReadRecord(xlogreader, RecPtr, &errormsg);
    |--  ReadRecPtr = xlogreader->ReadRecPtr;
  } while (record != NULL);
 
ShutdownXLOG->CreateRestartPoint:
  lastCheckPointRecPtr = XLogCtl->lastCheckPointRecPtr;
  ControlFile->checkPoint = lastCheckPointRecPtr;

1、备机回放

StartupXLOG
  do{
    ...
    RmgrTable[record->xl_rmid].rm_redo(xlogreader);//回放
    ...
    record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);//读取一个xlog
  } while (record != NULL);

2、回放函数

void
xlog_redo(XLogReaderState *record)
{
  ...
  else if (info == XLOG_CHECKPOINT_SHUTDOWN){
    ...
    memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
    ...
    RecoveryRestartPoint(&checkPoint);
  }else if (info == XLOG_CHECKPOINT_ONLINE){
    ...
    memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
    ...
    RecoveryRestartPoint(&checkPoint);
  }
  ...
}

3、RecoveryRestartPoint

static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
  ...
  SpinLockAcquire(&XLogCtl->info_lck);
  XLogCtl->lastCheckPointRecPtr = ReadRecPtr;//ReadRecPtr为读取checkpoint记录后的位置
  XLogCtl->lastCheckPointEndPtr = EndRecPtr;
  XLogCtl->lastCheckPoint = *checkPoint;
  SpinLockRelease(&XLogCtl->info_lck);
}

4、ReadRecPtr赋值

ReadRecord
  for (;;)
  {
    char     *errormsg;
 
    record = XLogReadRecord(xlogreader, RecPtr, &errormsg);
    ReadRecPtr = xlogreader->ReadRecPtr;
    EndRecPtr = xlogreader->EndRecPtr;
    ...
  }

5、备机createcheckpoint

bool
CreateRestartPoint(int flags)
{
 
  LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);
 
  /* Get a local copy of the last safe checkpoint record. */
  SpinLockAcquire(&XLogCtl->info_lck);
  lastCheckPointRecPtr = XLogCtl->lastCheckPointRecPtr;//checkpoint的位置来自XLogCtl->lastCheckPointRecPtr
  lastCheckPointEndPtr = XLogCtl->lastCheckPointEndPtr;
  lastCheckPoint = XLogCtl->lastCheckPoint;
  SpinLockRelease(&XLogCtl->info_lck);
 
  ...
 
  if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) || lastCheckPoint.redo <= ControlFile->checkPointCopy.redo){
    //回放了最后一个checkpoint记录后，备机再次手动执行checkpoint命令
    UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
    if (flags & CHECKPOINT_IS_SHUTDOWN){
      LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
      ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
      ControlFile->time = (pg_time_t) time(NULL);
      UpdateControlFile();
      LWLockRelease(ControlFileLock);
    }
    LWLockRelease(CheckpointLock);
    return false;
  }
  ...
  LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
  if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY && ControlFile->checkPointCopy.redo < lastCheckPoint.redo){
    ControlFile->prevCheckPoint = ControlFile->checkPoint;
    ControlFile->checkPoint = lastCheckPointRecPtr;//checkpoint的位置
    ControlFile->checkPointCopy = lastCheckPoint;
    ControlFile->time = (pg_time_t) time(NULL);
    ...
    if (flags & CHECKPOINT_IS_SHUTDOWN)
      ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
    UpdateControlFile();
  }
  ...
 
  return true;
}

6、备机shutdown

void
ShutdownXLOG(int code, Datum arg)
{
  /*
   * Signal walsenders to move to stopping state.
   */
  WalSndInitStopping();
 
  /*
   * Wait for WAL senders to be in stopping state.  This prevents commands
   * from writing new WAL.
   */
  WalSndWaitStopping();
 
  if (RecoveryInProgress())//备机写checkpoint
    CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
  else
  {
    /*
     * If archiving is enabled, rotate the last XLOG file so that all the
     * remaining records are archived (postmaster wakes up the archiver
     * process one more time at the end of shutdown). The checkpoint
     * record will go to the next XLOG file and won't be archived (yet).
     */
    if (XLogArchivingActive() && XLogArchiveCommandSet())
      RequestXLogSwitch(false);
 
    CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
  }
  ShutdownCLOG();
  ShutdownCommitTs();
  ShutdownSUBTRANS();
  ShutdownMultiXact();
}

7、总结

PostgreSQL备库也可以写检查点，目的是避免每次重启备库都需要从上一个检查点（由主库产生，在WAL中回放出来的）APPLY后面所有的WAL。但是他记录的checkpoint位点是从主库传过来的。这样的话就有问题了，如果主机很长时间都没有做checkpoint了，备机即使正常关闭，重启时，也会从上一个checkpoint开始恢复，这样也会恢复很长时间；并且多次重启也需要从上一次checkpoint开始重复恢复。

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原始发表：2019-04-19，如有侵权请联系 cloudcommunity@tencent.com 删除

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