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Sysbox Quick Start Guide: Storage

Sharing Storage Among System Containers

It's easy to share storage among multiple system containers by simply bind-mounting the shared storage into each system container.

  1. Create the shared storage on the host. In this example we use a Docker volume.
$ docker volume create shared-storage
shared-storage
  1. Create a system container and mount the shared storage volume into it:
$ docker run --runtime=sysbox-runc -it --rm --hostname syscont --mount source=shared-storage,target=/mnt/shared-storage alpine:latest
/ #
  1. From the system container, add a shared file to the shared storage:
/ # touch /mnt/shared-storage/shared-file
/ # ls -l /mnt/shared-storage/shared-file
-rw-r--r--    1 root     root             0 Oct 24 22:08 /mnt/shared-storage/shared-file
  1. In another shell, create another system container and mount the shared storage volume into it:
$ docker run --runtime=sysbox-runc -it --rm --hostname syscont2 --mount source=shared-storage,target=/mnt/shared-storage alpine:latest
/ #
  1. Confirm that the second system container sees the shared file:
/ # ls -l /mnt/shared-storage/shared-file
-rw-r--r--    1 root     root             0 Oct 24 22:08 /mnt/shared-storage/shared-file

Notice that both system containers see the shared file with root:root permissions, even though each system container is using the Linux user namespace with user-ID and group-ID mappings.

The reason both system containers see the correct root:root ownership on the shared storage is through the magic of the shiftfs filesystem, which Sysbox mounts over the shared storage.

From the first system container:

/ # mount | grep shared-storage
/var/lib/docker/volumes/shared-storage/_data on /mnt/shared-storage type shiftfs (rw,relatime)

In the example above we used a Docker volume as the shared storage. However, we can also use an arbitrary host directory as the shared storage. We need to simply bind-mount it to the system containers.

Finally, in this example we used shiftfs, but the same would apply to hosts with kernel >= 5.12 that use the alternative "ID-mapped mounts" mechanism.