Comment 3 for bug 1931758

Without the --storage arg, juju will use sensible (provider specific) defaults to provision the charm storage requirements. eg block storage on a lxd cloud will be a loop device, on aws it will be a ebs volume; a filesystem storage on lxd will be rootfs type, on aws it will be a fi;esystem on yop on an ebs volume.

All of the above is premised on there being the concept of a storage provisioner, some of which are built in to juju, like loop, rootfs, tmpfs; others are cloud specific, like ebs, cinder, k8s storage class etc.

k8s does support tmpfs and rootfs storage types - these are mapped to emytydir memory or disk respectively. But loop is not supported yet. And any filesystem juju creates on top of a block storage is hard wired to ext4 currently.

So yeah, juju could provision and provide a btrfs filesystem mounted at /foo and concourse should use that.