Debug Docker caching issues

Question

I am trying to improve CI/CD performance and part of that is improving the caching behaviour of Docker builds. I find myself trying to debug what exactly invalidates the cache. Are there any ways to query the Docker build process to get more information than "cached" and "not cached" ? For example, at some point, I add a large directory, which has not changed as far as I can tell, but busts the cache. I would like to find out what is causing that, exactly.

Answer 1

The docker cache needs the following:

• Build has been run on the same docker host previously (or you explicitly added flags to trust a pulled image) and
• The previous image needs to still be on the build host (not pruned) and
• The same previous layer and either
• The same command being run (including the same environment/args) or
• The same hash on the files being copied

If you see the cache being busted on a COPY or ADD command, and not the previous step, then you need to look at the hash being generated. All the files need to be identical, same file names, cases, file permissions, file owners, and the contents need to be bit for bit identical. You can look at the resulting checksum for this hash by running a docker image history command, e.g.:

$ docker image history golang:alpine
IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
6b21b4c6e7a3        3 weeks ago         /bin/sh -c #(nop) WORKDIR /go                   0B
<missing>           3 weeks ago         /bin/sh -c mkdir -p "$GOPATH/src" "$GOPATH/b…   0B
<missing>           3 weeks ago         /bin/sh -c #(nop)  ENV PATH=/go/bin:/usr/loc…   0B
<missing>           3 weeks ago         /bin/sh -c #(nop)  ENV GOPATH=/go               0B
<missing>           3 weeks ago         /bin/sh -c set -eux;  apk add --no-cache --v…   344MB
<missing>           3 weeks ago         /bin/sh -c #(nop)  ENV GOLANG_VERSION=1.12.7    0B
<missing>           3 weeks ago         /bin/sh -c [ ! -e /etc/nsswitch.conf ] && ec…   17B
<missing>           3 weeks ago         /bin/sh -c apk add --no-cache   ca-certifica…   551kB
<missing>           3 weeks ago         /bin/sh -c #(nop)  CMD ["/bin/sh"]              0B
<missing>           3 weeks ago         /bin/sh -c #(nop) ADD file:0eb5ea35741d23fe3…   5.58MB

You can include the --no-trunc option to get a full line. The key piece of data relevant to your question from the above image is in the ADD step where you see file:0eb5ea35741d23fe3…. If that checksum changes between two image builds, the cache will bust and you'll start a new set of layers.

Answer 2

According to Docker documentation search for: "Leverage build cache", there is checksum calculated on the directory - please see the article for details.

What I will suggest is a multistage build.

If we have static content of directory we could create container/image with its content. Then instead of using Docker copy, we could use FROM as a build stage and copy that directory into a new layer, which in fact will reuse that layer. That is based on a fact that we don't operate on a directory (when a file could get a changed attribute like last accessed time).

Base Dockerfile

FROM alpine:latest  
COPY /myDirectory .

Then you could tag this container as storage-base O

MultiStage Dockerfile

FROM storage-base AS storage

FROM alpine:latest  
RUN a command 
WORKDIR /root/
COPY --from=storage /myDirectory .

Please check more references here: Docker Multistage Build

Edit

1. the image/container created in step 1 shall be only recreated when directory content will change (by a trigger that knows that the data changed), so that means it will not be created on every build.
2. then we use the image that was created once (and re-use that layer)

I just created a simple example to help express my idea - Git repo - run build-docker.sh script

I found that on reboot Docker is re-creating cache instead of reusing, but it takes just a few minutes and it is not critical.

Answer 3

I have to deal with that same issue and the main problem is not the cache itself is the time that takes to make a build but also the problem of being sure you have the right changes on the image you are building.

I can recommend you some different options you can make separately or all together:

• Make cronjob that cleans your docker Daemon of old images periodically, for example, once a week. With that, you are sure to have fresh images every week and have a good performance with the cache for the daily build, the only one that takes a little bit more time is the first build after the flush.

• Make your own base image with all the content you know you don't change too often, for that you can have more than one Dockerfile on your project and launch a build process of the base image after the flush on the previous step to be sure you have the latest changes on a fresh image

• Use multi-stage build or build patter to have a good size image at the end of the build.

Answer 4

Did you try

docker inspect TAG

Also try the manual binary search method in the answer by Timmay to Server Fault StackExchange question "How to debug Docker cache invalidation?"

Answer 5

As I've discussed in this blog post, I found the following reasons for (unexpected) cache misses in Docker:

• If the problem happens in a CI pipeline, and if you use multiple build agent machines (or "ephemeral" machines), it might happen that build job #1 was executed on agent #A, but build job #2 was executed on agent #B, which has a different local cache than agent #A. Consequently, when you look at your CI pipeline output, be sure to check on which agent the jobs are executed. To prevent these kinds of cache misses, you can use a remote cache, storing caching information in a remote image registry. There are two implementation approaches: inline caching (where the image builder embeds caching meta-data into the image it builds), or using a separate registry cache (where a separate image is pushed that contains only cache blobs). The usage details of remote caching depend on your image builder tool. For instance, docker build supports only inline caching (see here), docker buildx (or when using BuildKit directly) supports both approaches (see here), Buildah and kaniko only support the registry cache.
• If you use ARG in your Dockerfile, it is easy to accidentally break the cache invalidation. Whenever the value of some ARG is different between two docker build executions, the second execution won’t be able to reuse the previously cached layer for a RUN or ENV command that uses the ARG's value. This then also invalidates all follow-up layers. See here for background information. If you use multi-stage builds, and if you run docker build several times (for different targets), make sure you always provide the same ARG values to all docker build calls!
• Sometimes the entire image is rebuilt whenever a new base image has been released (that you reference in a FROM statement). This particularly happens if you use docker build --pull. You need to closely look at the builder’s output of the first layer, which includes the SHA-256 checksum of the base image. If it keeps changing frequently, there is no real "fix". Your image should be rebuilt, to include the most recent security fixes of the base image. However, if the base image is rebuilt very often (e.g. multiple times per day), you may want to stop using the --pull flag, and instead have a different approach that only runs docker pull <base image> (or delete the base image) more rarely, e.g. once per day.
• Layers for COPY or ADD statements are rebuilt "unexpectedly" whenever files change that you did not have on your radar (which you did not include into your .dockerignore yet). This could be the .git folder, or files created during building/testing (e.g. unit test report files, or log files). This typically happens when running COPY . ., because then your entire project directory is copied from the build context into the build container, which increases the chance that you missed excluding some superfluous files (that do not belong into the container anyway) via .dockerignore. To make it easy to address this issue, I developed a small CLI tool called Directory Checksum. It recursively computes the checksum of the contents of a directory, and prints the checksums up to a depth you can specify.