Cntr: Lightweight OS Containers

Container-based virtualization has become the de-facto standard for deploying applications in data centers. However, deployed containers frequently include a wide-range of tools (e.g., debuggers) that are not required for applications in the common use-case, but they are included for rare occasions such as in-production debugging. As a consequence, containers are significantly larger than necessary for the common case, thus increasing the build and deployment time. CNTR provides the performance benefits of lightweight containers and the functionality of large containers by splitting the traditional container image into two parts: the "fat" image--containing the tools, and the "slim" image -- containing the main application. At run-time, CNTR allows the user to efficiently deploy the "slim" image and then expand it with additional tools, when and if necessary, by dynamically attaching the "fat" image. To achieve this, CNTR transparently combines the two container images using a new nested namespace, without any modification to the application, the container manager, or the operating system. We have implemented CNTR in Rust, using FUSE, and incorporated a range of optimizations. CNTR supports the full Linux filesystem API, and it is compatible with all container implementations (i.e., Docker, rkt, LXC, systemd-nspawn). Through extensive evaluation, we show that CNTR incurs reasonable performance overhead while reducing, on average, by 66.6% the image size of the Top-50 images available on Docker Hub.