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Merge pull request #20672 from dvdksn/add-copy-cache-mtime 

build: mtime doesnt invalidate cache
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David Karlsson 2024-09-02 13:10:45 +02:00 committed by GitHub
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content/build/cache/invalidation.md vendored
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@ -5,27 +5,32 @@ keywords: build, buildx, buildkit, cache, invalidation, cache miss
--- ---
When building an image, Docker steps through the instructions in your When building an image, Docker steps through the instructions in your
Dockerfile, executing each in the order specified. For each instruction, Docker Dockerfile, executing each in the order specified. For each instruction, the
checks whether it can reuse the instruction from the build cache. [builder](/build/builders/_index.md) checks whether it can reuse the
instruction from the build cache.
## General rules ## General rules
The basic rules of build cache invalidation are as follows: The basic rules of build cache invalidation are as follows:
- Starting with a base image that's already in the cache, the next - The builder begins by checking if the base image is already cached. Each
instruction is compared against all child images derived from that base subsequent instruction is compared against the cached layers. If no cached
image to see if one of them was built using the exact same instruction. If layer matches the instruction exactly, the cache is invalidated.
not, the cache is invalidated.
- In most cases, simply comparing the instruction in the Dockerfile with one - In most cases, comparing the Dockerfile instruction with the corresponding
of the child images is sufficient. However, certain instructions require more cached layer is sufficient. However, some instructions require additional
examination and explanation. checks and explanations.
- For the `ADD` and `COPY` instructions, the modification time and size file - For the `ADD` and `COPY` instructions, and for `RUN` instructions with bind
metadata is used to determine whether cache is valid. During cache lookup, mounts (`RUN --mount=type=bind`), the builder calculates a cache checksum
from file metadata to determine whether cache is valid. During cache lookup,
cache is invalidated if the file metadata has changed for any of the files cache is invalidated if the file metadata has changed for any of the files
involved. involved.
The modification time of a file (`mtime`) is not taken into account when
calculating the cache checksum. If only the `mtime` of the copied files have
changed, the cache is not invalidated.
- Aside from the `ADD` and `COPY` commands, cache checking doesn't look at the - Aside from the `ADD` and `COPY` commands, cache checking doesn't look at the
files in the container to determine a cache match. For example, when processing files in the container to determine a cache match. For example, when processing
a `RUN apt-get -y update` command the files updated in the container a `RUN apt-get -y update` command the files updated in the container