Splitlog() executed while the namenode was in safemode may cause data-loss

[bijieshan]

Hi,
I found this problem while the namenode went into safemode due to some unclear 
reasons. 
There's one patch about this problem:

   try {
      HLogSplitter splitter = HLogSplitter.createLogSplitter(
        conf, rootdir, logDir, oldLogDir, this.fs);
      try {
        splitter.splitLog();
      } catch (OrphanHLogAfterSplitException e) {
        LOG.warn("Retrying splitting because of:", e);
        // An HLogSplitter instance can only be used once.  Get new instance.
        splitter = HLogSplitter.createLogSplitter(conf, rootdir, logDir,
          oldLogDir, this.fs);
        splitter.splitLog();
      }
      splitTime = splitter.getTime();
      splitLogSize = splitter.getSize();
    } catch (IOException e) {
      checkFileSystem();
      LOG.error("Failed splitting " + logDir.toString(), e);
      master.abort("Shutting down HBase cluster: Failed splitting hlog 
files...", e);
    } finally {
      this.splitLogLock.unlock();
    }

And it was really give some useful help to some extent, while the namenode 
process exited or been killed, but not considered the Namenode safemode 
exception.
   I think the root reason is the method of checkFileSystem().
   It gives out an method to check whether the HDFS works normally(Read and 
write could be success), and that maybe the original propose of this method. 
This's how this method implements:

    DistributedFileSystem dfs = (DistributedFileSystem) fs;
    try {
      if (dfs.exists(new Path("/"))) {  
        return;
      }
    } catch (IOException e) {
      exception = RemoteExceptionHandler.checkIOException(e);
    }
   
   I have check the hdfs code, and learned that while the namenode was in 
safemode ,the dfs.exists(new Path("/")) returned true. Because the file system 
could provide read-only service. So this method just checks the dfs whether 
could be read. I
think it's not reasonable.
    
    
Regards,
Jieshan Bean