public void initializeWithExistingSlice(ByteBuffer byteBuffer, int allocSize) {
    this.byteBuffer = byteBuffer;
    this.allocSize = allocSize;
  }
Allocator uses this to remember the allocation size. Somewhat redundant with header.
llap-server/src/java/org/apache/hadoop/hive/llap/cache/LlapAllocatorBuffer.java