void initBufferForAppend() throws StoreException {
    if (byteBufferForAppend != null) {
      throw new StoreException("ByteBufferForAppend has been initialized.", StoreErrorCodes.InitializationError);
    }
    byteBufferForAppend = ByteBuffer.allocateDirect(BYTE_BUFFER_SIZE_FOR_APPEND);
    byteBufferForAppendTotalCount.incrementAndGet();
  }
Initialize a direct {@link ByteBuffer} for {@link LogSegment#appendFrom(ReadableByteChannel, long)}.
The buffer is to optimize JDK 8KB small IO write.
ambry-store/src/main/java/com/github/ambry/store/LogSegment.java