[Bug 1594849] Re: systemd-nspawn: option --overlay not working

[Olaf Dietsche]

It seems, you're right with the final path component in --overlay. This
is the absolute path *inside* the container. I looked into nspawn's
source code and the options is interpreted as follows

    --overlay=lo1:lo2:up:dest

lo1 and lo2 are lower directories, up is the upper directory in
overlayfs terms, and all are absolute host directory paths. dest is the
where the overlayfs is mounted inside the container's root directory.

In order to use --overlay, I have to say

--overlay=/path/to/xenial/usr:/path/to/apache/usr:/path/to/container/usr:/usr

and repeat this for /etc, /bin, /sbin, /lib, /lib64, /var.

So --overlay seems to work, sort of.

Anyway, thank you for pushing me into the right direction. I think, I'll
stay with my workaround, overlay mounting the base directories upfront,
and then nspawning the container on the resulting root directory.

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Title:
  systemd-nspawn: option --overlay not working

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  I have a directory with a minimal Xenial installation, which I want to
  share among several containers. I also have a directory with just
  Apache and its dependencies installed.

  systemd-nspawn has an option --overlay to "Combine multiple directory
  trees into one overlay file system and mount it into the container."

  Neither `systemd-nspawn --overlay=/path/to/xenial:/path/to/apache -D
  /path/to/container` nor `systemd-nspawn
  --overlay=/path/to/xenial:/path/to/apache:/path/to/container -D
  /path/to/container` works as expected.

  Both report an error "Directory /path/to/container doesn't look like
  it has an OS tree. Refusing.". Of course, it doesn't have an OS tree,
  this is what the base overlays are for.

  Looking at the source code, creating a subdirectory
  /path/to/container/usr works around this. But now it complains about
  "Failed to create directory /path/to/container/sys/fs/selinux: Read-
  only file system" twice, then about a missing timezone, and finally
  aborts with

  Creating mount point for overlay /path/to/container/path/to/apache
  failed: No such file or directory

  Doing this manually instead, works fine:

  mount -t overlay -o lowerdir=/path/to/apache:/path/to/xenial,upperdir=/path/to/container,workdir=/path/to/workdir none /path/to/container
  systemd-nspawn -D /path/to/container

  1)
  Description:	Ubuntu 16.04 LTS
  Release:	16.04

  2)
  systemd-container:
    Installed: 229-4ubuntu6
    Candidate: 229-4ubuntu6
    Version table:
   *** 229-4ubuntu6 500
          500 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages
          100 /var/lib/dpkg/status
       229-4ubuntu4 500
          500 http://de.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages

  3)
  I expect systemd-nspawn to mount an overlayfs with the provided directories and then start the container with all the directories combined.

  4)
  systemd-nspawn complains

  - Directory /path/to/container doesn't look like it has an OS tree.
  Refusing.

  and after working around this (among other irrelevant error messages)

  - Creating mount point for overlay /path/to/container/path/to/apache
  failed: No such file or directory

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