Can't create a snapshot of a Btrfs subvolume

[Serge Hallyn]

Quoting Martín Cigorraga (martincigorraga at gmail.com):
> Hi!
> 
> I recently installed an Ubuntu Server with this layout:
> partition 1: /          Btrfs
> partition 2: /home  Btrfs
> partition 3: swap
> and I'm unable to create any snapshot of / -- a problem I never had before
> when installing with the default Btrfs layout (that is, with both / and
> /home subvolumes on the same partition).
> 
> My current Btrfs layout is this one:
> UUID=blah-blah-blah /               btrfs
> defaults,space_cache,autodefrag,relatime,compress-force=zlib,subvol=@
>   0       1
> UUID=blah-blah-blah /home       btrfs
> defaults,space_cache,autodefrag,relatime,compress-force=zlib,subvol=@home 0
>       2
> UUID=blah-blah-blah none         swap  sw
> 
>  0       0
> 
> Now, when I try to create a snapshot I get this:
> ~> sudo btrfs subvolume snapshot -r / /home/SYSTEM/snapshots/root
> Create a readonly snapshot of '/' in '/home/SYSTEM/snapshots/root'
> ERROR: cannot snapshot '/' - Invalid argument
> (Note: /home/SYSTEM/snapshots directory already exists)
> This is strange because the same exact command does work as expected when /
> and /home subvolumes are in the same partition.

Hi,

I could be off base, but I would expect precisely this result.  I would
not expect btrfs subvolume to be able to create a subvolume in a different
filesystem from the origin subvolume.  You could probably use 'btrfs
send/receive' for that, but I've not used those.

Can you just keep your rootfs backups under something like /BACKUPS?
(Keeping in mind of course they won't include /home contents)

-serge