void writeBlock(BlockWritable bw, FSDataOutputStream out) throws IOException {
      bw.writeToBlock(startWriting(bw.getBlockType()));
      writeHeaderAndData(out);
    }
Takes the given {@link BlockWritable} instance, creates a new block of its appropriate type,
writes the writable into this block, and flushes the block into the output stream. The writer
is instructed not to buffer uncompressed bytes for cache-on-write.
@param bw  the block-writable object to write as a block
@param out the file system output stream
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java