lxc-create -B btrfs fails if lxc.lxcpath is not on the same filesystem as /var/cache/lxc (not a configurable option)

[Shawn Teague]

## Version
Description: Ubuntu 15.04
Release: 15.04
lxc:
  Installed: 1.1.2-0ubuntu3.1
  Candidate: 1.1.2-0ubuntu3.1
  Version table:
 *** 1.1.2-0ubuntu3.1 0
        500 http://us.archive.ubuntu.com/ubuntu/ vivid-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu/ vivid-security/main amd64 Packages
        100 /var/lib/dpkg/status
     1.1.2-0ubuntu3 0
        500 http://us.archive.ubuntu.com/ubuntu/ vivid/main amd64 Packages

## Config changes from default
>cat /etc/lxc/lxc.conf
lxc.lxcpath = /srv/vservers/lxc

## Mount details
/var/cache/lxc in on root filesystem
/srv/vservers/lxc is a btrfs mount point

## Command used (sample)
> sudo lxc-create -B btrfs --name test-bt -t ubuntu -- -r trusty

## Error (sample)
Copy /var/cache/lxc/trusty/rootfs-amd64 to /srv/vservers/lxc/test-bt/rootfs ...
Copying rootfs to /srv/vservers/lxc/test-bt/rootfs ...
Transaction commit: none (default)
Delete subvolume '/srv/vservers/lxc/test-bt/rootfs'
Create a snapshot of '/var/cache/lxc/trusty/rootfs-amd64' in '/srv/vservers/lxc/test-bt/rootfs'
ERROR: cannot snapshot '/var/cache/lxc/trusty/rootfs-amd64' - Invalid cross-device link
lxc_container: lxccontainer.c: create_run_template: 1108 container creation template for test-bt failed
lxc_container: lxccontainer.c: container_destroy: 2056 Error destroying rootfs for test-bt
lxc_container: lxc_create.c: main: 274 Error creating container test-bt

## Thoughts
Maybe this is the expected behavior, if so a Documentation update stating something to the effect of "If you change the lxcpath to a different filesystem than the one where /var/cache/lxc livs you cannot create a btrfs backed container."
-or-
Let /var/cache/lxc be configured via lxc.conf and have the documentation note that they should reside on the same filesystem.

[Serge Hallyn (serge-hallyn)]

Indeed this is expected behavior. Since there is no way to
create a btrfs subvolume in this instance, and you explicitly
asked for a btrfs subvolume, it should error. A simple directory
copy must be used.

I think it might be reasonable to say that even if it has to
do a full directory copy, it could still create a new subvolume
so that clones can be done as snapshots. Patches for either that
or for documentation improvement are welcome, so leaving this bug
open

[Christian Brauner (cbrauner)]

Marking as won't fix since LXC 1.1.* is EOL. Also, there have been quite some improvements in btrfs handling on LXC side. If the issue still persists, please feel free to report a bug against or repo on github.