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Why does `chmod` cause `docker build` to run out of disk space – CentOS

so my Docker build is essentially done and I just have to chown and chmod a large directory recursively.

The following is part of the output from docker build:

Step 34/40 : RUN du -b -m -s / 2>/dev/null || true
 ---> Running in d261dfc7a9f8
1537    /
Removing intermediate container d261dfc7a9f8
 ---> 663d129f1487
Step 35/40 : RUN df --si -m /;df -i /;
 ---> Running in 8249096d3069
Filesystem     1M-blocks  Used Available Use% Mounted on
overlay            29674 28609      1066  97% /
Filesystem           Inodes  IUsed   IFree IUse% Mounted on
overlay             2737520 549839 2187681   21% /
Removing intermediate container 8249096d3069
 ---> fc4250e3433d
Step 36/40 : RUN chmod -R a+rwX /var/www/html
 ---> Running in 2705265de405
Error processing tar file(exit status 1): open /var/www/html/[...].php: no space left on device

I solved my issue by running docker system prune -a, to free up an extra 908 MB, but I don’t understand why disk space was an issue in the first place (not to mention what tar has to do with it…).

Running watch df -m / I can clearly see that chmod is resulting in large swings of several 100MBs (down and up), but that doesn’t make any sense since chmod should not change disk allocation, not even with CoW (copy on write) files or sparse files.

Why does dockers implementation of chmod (I’m using FROM centos:7.4.1708) cause extra disk space to be used?

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2

Answers


  1. 0

    It’s not chmod that is causing the extra disk space. The run command in Dockerfile creates a layer caching that will use up your disk space. I would suggest to aggregate the commands you run using &&.

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  2. 0

    Filesystem layers in docker are implemented with a copy-on-write solution. Any change to a file results in a copy of that file first, and then the change is applied to that file copy, even a permission, owner, or timestamp. The chown and chmod commands, when run recursively, will change timestamps on the file, even if no permissions are changed.

    Therefore to minimize the size of layers in docker, make all changes to a file in the same layer. Each step creates a new layer, so consolidate changes into the same step. With a COPY command, fix the permissions on the source and see the options to adjust ownership. And with RUN, you will often see commands chained with the && syntax to compact into a single step. This is particularly important when creating temporary files, since you need to delete them before the step finishes to avoid writing them into the image layer.

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