code: public BlobStoreFile write(String key, boolean create) throws IOException {
          return new HdfsBlobStoreFile(getKeyDir(key), true, create, _hadoopConf);
      }
docs: Get an object tied to writing the data.
  
  @param key    the key of the part to write to.
  @param create whether the file needs to be new or not.
  @return an object that can be used to both write to, but also commit/cancel the operation.
  @throws IOException on any error