Shared-Subtree Concept, Implementation and Applications in Linux

Concepts like per-process-namespaces and bind mounts have enriched VFS for a couple of years now. Various solutions have attempted to use these features for customized mount setup in a virtualised environment and for setting up mirrored mount trees, to support version-ed filesystem.

But more than often the static nature of per-process-namespace and bind-mounts have restricted their use. We introduced a new VFS enhancement in 2.6.15 kernel called shared subtree. This enhancement makes the per-process-namespace and bind mounts dynamic in nature and provides a crucial building block for various solutions.

We discuss the feature semantics and its applications in real life. We also discuss the design and implementation detail of this feature.