What does the sync and syncFs of SequenceFile.Writer means?

Yes, probably.

sync() create a sync point. As stated in the book "Hadoop- The Definitive Guide" by Tom White (Cloudera)

a sync point is a point in the stream which can used by to resynchronize with a record boundary if the reader is "lost" - for example after seeking to an arbitrary position on the stream.

Now the implementation of syncFS() is pretty simple:

   public void syncFs() throws IOException {      if (out != null) {        out.sync();                               // flush contents to file system      }    }

where out is a FSDataOutputStream. Again, in the same book is stated:

HDFS provides a method for forcing all buffers to be synchronized to the datanodes via the sync() method on FSDataOutputStream. After a successful call return from sync() HDFS garantees that the data written up to that point in the file is persisted and visible to all readers. In the event of a crash (of the client or HDFS), the data will not be lost.

But a footnote warns to look to bug HDFS-200, since the visibility mentioned above was not always not always honored.