ceph rbd：cache

以aio_write为例，给出librbd cache相关的调用顺序：

注意，下文用 -> 表示实例函数 ，::表示静态函数。

• Image->aio_write
  • ImageRequestWQ->aio_write
    • ImageRequest::aio_write
      • ImageRequest->send
        • if (m_bypass_image_cache || m_image_ctx.image_cache==nullptr)
          • AbstractImageWriteRequest->send_request
        • else
          • ImageWriteRequest->send_image_cache_request
            • PassthroughImageCache->aio_write
              • ImageWriteback->aio_write
                • ImageWriteRequest->set_bypass_image_cache
                • ImageWriteRequest->send
                  • AbstractImageWriteRequest->send_request
                    * if (image_ctx.object_cacher == nullptr)
                    * AbstractImageWriteRequest->send_object_requests
                    * else
                    * ImageWriteRequest->send_object_cache_requests
                    * ImageCtx->write_to_cache