Setup-Storage: how to specify multiple btrfs subvolumes?

[Urs Ganse]

Dear FAI people,

I have a question about the recently added btrfs support, which is not 
exactly clear to me from the existing documentation:

I am installing systems with a single disk, of which one partition is 
supposed to contain a btrfs file system.
To make updating easier, I'd like to have multiple subvolumes in that 
filesystem, especially I'd like to have the system root in a non-/ 
default subvolume (so that later on, I can snapshot that subvolume to 
perform easily-rollback-able upgrades).

How do I specify such a configuration in the setup-storage config? The 
documentation only gives an example for btrfs lines with subvolumes that 
use raid configurations, but are not exactly clear about how to specify 
multiple subvolumes in the same partition. Also, is there a way to 
specify the default subvolume here?

The formal config file syntax description is missing any mention of the 
btrfs lines, too.

Cheers,

//Urs