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Module layer

composefs_storage

Module layer 

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Layer reading and metadata handling.

This module provides access to individual overlay layers and their metadata. Layers are the fundamental storage units in the overlay driver, representing filesystem changes that are stacked to form complete container images.

§Overview

The Layer struct represents a single layer in the overlay filesystem. Each layer contains:

  • A diff/ directory with the actual file contents
  • A link file containing a short 26-character identifier
  • A lower file listing parent layers (if not a base layer)
  • Metadata for whiteouts and opaque directories

§Layer Structure

Each layer is stored in overlay/<layer-id>/:

overlay/<layer-id>/
+-- diff/                 # Layer file contents
|   +-- etc/
|   |   +-- hosts
|   +-- usr/
|       +-- bin/
+-- link                  # Short link ID (26 chars)
+-- lower                 # Parent references: "l/<link-id>:l/<link-id>:..."

§Whiteouts and Opaque Directories

The overlay driver uses special markers to indicate file deletions:

  • .wh.<filename> - Whiteout file (marks <filename> as deleted)
  • .wh..wh..opq - Opaque directory marker (hides lower layer contents)

Structs§

Layer
Represents an overlay layer with its metadata and content.