Nesting Overlay File Systems with ShadowWhiteout
Kenta Ishiguro, Kohei Hayama, Ayase Yokoyama, Hiroshi Yamada, Toshio Hirotsu · 2025
Container technology improves storage efficiency by sharing base images through overlay file systems (overlayfs). Overlayfs enables temporary modifications while preserving original files by merging multiple directories into a single unified view. However, in container-in-container environments, the current overlayfs approach incurs inefficiency in terms of storage usage, as it does not support nesting due to file deletion operations. To delete a preserved original file, a special file is created indicating that the file is deleted in the overlayfs, but this mechanism lacks expressiveness for nesting. This paper introduces ShadowWhiteout, a new representation for file deletion in overlayfs. We identify key technical challenges and implement a ShadowWhiteout-capable overlayfs. Our system allows nesting overlayfs while maintaining reasonable runtime overhead.