[systemd-devel] [HEADS-UP] Discoverable Partitions Spec

[Alex Elsayed]

Lennart Poettering wrote:

> On Mon, 10.03.14 23:39, Goffredo Baroncelli (kreijack at libero.it) wrote:
> 
>> > Well, the name is property of the admin really. There needs to be a way
>> > how the admin can label his subvolumes, with a potentially localized
>> > name. This makes it unsuitable for our purpose, we cannot just take
>> > possession of this and leave the admin with nothing.
>> 
>> Instead of the name we can use the xattr to store these information.
> 
> Ah, using xattrs for this is indeed an option. That way we should be able
> attach any kind of information we like to a subvolume.
> 
> Hmm, I figure though that there is no way currently to read xattrs off a
> subvolume without first mounting them individually? Having to mount all
> subvolumes before we can make sense of them and mount them to the right
> place certainly sounds less than ideal...

Well, you can always mount subvol=. aka subvolid=0 - the 'root subvolume' 
since they are strictly hierarchical. 'btrfs subvolume list' can then give 
you every subvol in the FS.

>> > On GPT there are also gpt partition labels and partition types. The
>> > former are property of the admin, he can place there whatever he wants,
>> > in whatever language he chooses... The latter however is how we make
>> > sense of it on a semantical level.
>> > 
>> >> Or in another way we could group the different systems in
>> >> subdirectories:
>> >>
>> >> @home		-> home of all the systems
>> >> @srv		-> srv  of all the systems
>> >> fedora/@	-> root of a fedora system
>> >> fedora/@etc	-> etc of the fedora system
>> >> fedora2/@	-> root of a fedora2 system
>> >> fedora2/@etc	-> etc of the fedora2 system
>> > 
>> > I am pretty sure automatic discovery of mount points should not cover
>> > the usecase where people install multiple distributions into the same
>> > btrfs volume. THe automatic logic should cover the simple cases only,
>> > and it sounds way over the top to support installing multiple OSes into
>> > the same btrfs... I mean, people can do that, if they want to, they
>> > just have to write a proper fstab, which I think is not too much too
>> > ask...
>> 
>> In your specification, you referred the use case of "container" (via
>> nspawn / libvrt-lxc). which have to boot "a disk image". Why you don't
>> mind to use a container on a btrfs snapshot ? I think that it will be
>> reasonable to have different containers on a snapshots of the same
>> filesystem-tree.
> 
> Hmm, dunno, you might have a point there...

I can confirm that I _currently_ do btrfs-subvol based containers with 
libvirt-lxc and it's quite useful... and in terms of on-disk hierarchy, each 
machine is a subvol at the toplevel in subvolid=0. Equal to the host.

i.e.:
subvol=.
  virt-host
  postgres
  powerdns
  ...etc...

Where postgres and powerdns are LXC container filesystems.

> Lennart
>