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Cloud and Container Basics

Inside a container started from an image, the configuration file /etc/app.conf, which is part of the image, is modified. Which of the following correctly describes what happens with Docker's usual storage mechanism (such as overlay2)?

  1. AThe file is copied into the container's writable layer and then modified, so the image and other containers are not affected
  2. BThe file in the image layer is modified directly, so other containers started from the same image also see the new contents
  3. CImage layers are read-only, so files contained in the image cannot be modified, and the write fails with an error
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Answer: A. The file is copied into the container's writable layer and then modified, so the image and other containers are not affected

A container's file system presents the read-only image layers with a writable layer for that container stacked on top, as a single tree. The first time a file in a lower layer is modified, it is copied into the writable layer, and from then on the copy is used (copy-on-write). The image layers therefore stay unchanged, and other containers sharing the same image are not affected. Because the first copy of a large file takes time, data that is written frequently, such as database data, is placed in a volume.

  • A. Correct. On the first write, the file is copied into the writable layer.
  • B. Image layers are read-only and shared by several containers, so they are not modified directly.
  • C. The write does not fail; it happens after the file is copied into the writable layer.
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