Copy-on-write

As explained in Kubernetes in Action, Second Edition

A copy-on-write mechanism preserves shared data until a modification causes a private copy to be created.

Copy-on-write (CoW) preserves shared container image files until an application changes one. The filesystem combines read-only image layers with a private read/write layer. section 2.1.2

Changing a shared file

Containers A and B can read the same image-layer files. If A changes a file, Docker copies the entire file into A’s read/write layer and changes that copy, so B remains unaffected. Deleting a file marks it deleted above the lower layer rather than removing it from the image. section 2.1.2

Account for these operational consequences:

• Changing permissions or ownership also copies the entire file, so modifying large or numerous files can significantly increase writable-layer usage. section 2.1.2
• Each container has its own writable layer, which isolates changes even when containers share image layers. section 2.1.2

Figure 2.8 Containers can share image layers.

Use the layer diagram to trace how containers share image data while retaining separate writable changes. section 2.1.2

How Kubernetes in Action, Second Edition covers it

How deep it goes

Explains layered reads, whole-file copying, deletion markers, and storage consequences without detailing filesystem-driver internals.

Examples

Uses three containers sharing layers, file modification, file deletion, and permission changes as concrete cases.

What you can do after

Evaluate how container file operations affect isolation and writable-layer storage.

How it connects

Connects container images, image layers, container filesystems, writable layers, and container isolation.

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Also explained in

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  liveBookmanning.com
  Grokking SimplicityA data-management technique that creates a copy before modifying data so the original remains unchanged.
• 

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  Architecting an Apache Iceberg LakehouseCopy-on-write is a data-modification pattern that rewrites affected base files immediately to materialize updates or deletions.
• 

  liveBookmanning.com
  Learn Docker in a Month of Lunches, Second EditionCopy-on-write is a storage technique that copies shared data into a writable location only when a modification is made.
• 

  liveBookmanning.com
  The Joy of JavaScriptA mutation strategy in which changes are made by creating modified copies rather than altering an existing value in place.

Compare how 5 books explain copy-on-write →

Related concepts

• shallow copyuses · A shallow copy duplicates a data structure’s outer container while retaining references to its nested values rather than copying them recursively.
• immutable collectionimplements · An immutable collection is a collection whose membership and contents cannot be changed after creation.
• Cowimplemented by · Cow is Rust’s standard-library clone-on-write type, representing either borrowed or owned data and creating owned data when necessary.
• writable layeruses · A writable layer is a mutable, container-specific filesystem layer that records changes made above read-only image layers.
• snapshotused by · A snapshot is a fixed representation or copy of data or system state as it existed at a particular point in time.

This explainer was generated by Manning's knowledge graph from the text of Kubernetes in Action, Second Edition. The section links point to the parts of the book it draws on.