code: public void write(WritableByteChannel channel) throws IOException
      {
          long totalSize = totalSize();
          RandomAccessReader file = sstable.openDataReader();
          ChecksumValidator validator = new File(sstable.descriptor.filenameFor(Component.CRC)).exists()
                                      ? DataIntegrityMetadata.checksumValidator(sstable.descriptor)
                                      : null;
          transferBuffer = validator == null ? new byte[DEFAULT_CHUNK_SIZE] : new byte[validator.chunkSize];
  
          // setting up data compression stream
          compressedOutput = new LZFOutputStream(Channels.newOutputStream(channel));
          long progress = 0L;
  
          try
          {
              // stream each of the required sections of the file
              for (Pair<Long, Long> section : sections)
              {
                  long start = validator == null ? section.left : validator.chunkStart(section.left);
                  int readOffset = (int) (section.left - start);
                  // seek to the beginning of the section
                  file.seek(start);
                  if (validator != null)
                      validator.seek(start);
  
                  // length of the section to read
                  long length = section.right - start;
                  // tracks write progress
                  long bytesRead = 0;
                  while (bytesRead < length)
                  {
                      long lastBytesRead = write(file, validator, readOffset, length, bytesRead);
                      bytesRead += lastBytesRead;
                      progress += (lastBytesRead - readOffset);
                      session.progress(sstable.descriptor, ProgressInfo.Direction.OUT, progress, totalSize);
                      readOffset = 0;
                  }
  
                  // make sure that current section is send
                  compressedOutput.flush();
              }
          }
          finally
          {
              // no matter what happens close file
              FileUtils.closeQuietly(file);
              FileUtils.closeQuietly(validator);
          }
      }
docs: Stream file of specified sections to given channel.
  
  StreamWriter uses LZF compression on wire to decrease size to transfer.
  
  @param channel where this writes data to
  @throws IOException on any I/O error