code: public void logSyncAll() throws IOException {
      // Record the most recent transaction ID as our own id
      synchronized (this) {
        TransactionId id = myTransactionId.get();
        id.txid = txid;
      }
      // Then make sure we're synced up to this point
      logSync();
    }
docs: Blocks until all ongoing edits have been synced to disk. This differs from
  logSync in that it waits for edits that have been written by other threads,
  not just edits from the calling thread.
  
  NOTE: this should be done while holding the FSNamesystem lock, or else more
  operations can start writing while this is in progress.