The default 8KB buffer of BlockReaderRemote#newBlockReader#BufferedOutputStream is too big

public static BlockReader newBlockReader(String file,
    ExtendedBlock block,
    Token<BlockTokenIdentifier> blockToken,
    long startOffset, long len,
    boolean verifyChecksum,
    String clientName,
    Peer peer, DatanodeID datanodeID,
    PeerCache peerCache,
    CachingStrategy cachingStrategy,
    int networkDistance) throws IOException {
  // in and out will be closed when sock is closed (by the caller)
  final DataOutputStream out = new DataOutputStream(new BufferedOutputStream(
      peer.getOutputStream()));
  new Sender(out).readBlock(block, blockToken, clientName, startOffset, len,
      verifyChecksum, cachingStrategy);
}

public BufferedOutputStream(OutputStream out) {
    this(out, 8192);
}