OverlayFS

overlayFS is a union mount filesystem for Linux that merges a writable upper directory with one or more read-only lower directories to present a single coherent view. It enables copy-on-write behavior, so changes go into the upper layer while the lower layers remain intact. The filesystem is implemented in the Linux kernel and is commonly used by container runtimes and sandboxing tools.

Architecture and operation: The core elements are lowerdir (read-only), upperdir (writable), and workdir (an auxiliary writable directory used by the implementation). When a file in the merged tree is modified, the data is copied from the lower layer into the upper layer (copy-up). Deletions in the merged view are represented by whiteout entries in the upper layer to hide lower-layer files from the merged view.

Variants and adoption: The original driver is referred to as overlay, and a more scalable variant called overlay2 is widely used by container platforms such as Docker. Overlay2 supports multiple lower layers and is generally preferred on modern systems. Both require a kernel with OverlayFS support and a compatible backing filesystem such as ext4 or XFS.

Limitations and considerations: OverlayFS relies on lower, upper, and work directories residing on the same filesystem; performance and semantics can vary with kernel version and filesystem type. Some metadata operations and special file types may behave differently across layers, and older kernels or filesystems may impose restrictions on features like opaque directories and whiteouts.