CODEWITHSUNDEEP
dockerdocker-volume
weberc2
weberc2

Reputation: 7908

Mount read-write volume inside of read-only volume

I have two volumes: repo and cache. I want to mount repo read-only at /repo and I want to mount cache as read-write at /repo/cache. When the app writes to /repo/cache, I expect the writes to go to the cache volume, while the /repo volume is not changed; however, when I run docker run -it -v repo:/repo:ro -v cache:/repo/cache alpine, I get docker: Error response from daemon: OCI runtime create failed: container_linux.go:345: starting container process caused "process_linux.go:430: container init caused \"rootfs_linux.go:58: mounting \\\"/var/lib/docker/volumes/cache/_data\\\" to rootfs \\\"/var/lib/docker/overlay2/9f1dfff7f943921fd2da278d357bec88007d7fcb41eb9bc34fc0e728c459ad73/merged\\\" at \\\"/var/lib/docker/overlay2/9f1dfff7f943921fd2da278d357bec88007d7fcb41eb9bc34fc0e728c459ad73/merged/repo/cache\\\" caused \\\"mkdir /var/lib/docker/overlay2/9f1dfff7f943921fd2da278d357bec88007d7fcb41eb9bc34fc0e728c459ad73/merged/repo/cache: read-only file system\\\"\"": unknown.

Is there a better way to achieve this result? I need the repo volume to be ro so the app doesn't write to it, and I need cache to be writable and mounted "inside" of /repo because that's where the app expects it (and I can't modify the app to look elsewhere).

Upvotes: 0

Views: 1307

Answers (1)

larsks
larsks

Reputation: 311635

To mount your cache volume on /repo/cache, Docker would need to be able to create a directory /repo/cache inside the container. Since you have mounted the repo volume read-only, this isn't possible.

If you first create the mountpoint yourself:

docker run --rm -v repo:/repo alpine mkdir /repo/cache

Then you will be able to successfully mount /repo/cache despite repo being read-only:

bash-5.0$ docker run --rm -it -v repo:/repo:ro -v cache:/repo/cache alpine
/ # ls /repo
cache

Upvotes: 4

Related Questions