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

[Lennart Poettering]

On Fri, 07.03.14 20:47, Mantas Mikulėnas (grawity at gmail.com) wrote:

> 
> On Fri, Mar 7, 2014 at 8:26 PM, Lennart Poettering
> <lennart at poettering.net> wrote:
> > Heya!
> >
> > Since yesterday systemd in git can now discover root, /home, /srv and
> > swap partitions automatically based on GPT type GUIDs, thus making
> > /etc/fstab unnecessary for simple setups.
> >
> > I have now put together something like a spec describing the logic
> > behind that, and what it is good for:
> >
> > http://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/
> >
> > It would be good if in the long run OS installers could adopt this and
> > use the right partition type GUIDs automatically, to make this discovery
> > work. For now however, you need to manually change the GPT type GUIDs of
> > your installation if you want to make use of this scheme.
> 
> That might not work very well if one tried to dual-boot two systemd
> distros… 

Hmm? there are all kinds of provisions in the spec to make sure this
works correctly. For example, we won't do this all for /usr and /var and
/etc since we cannot allow incorrect mixing and matching between parallel
installations. However, for /home and /srv that is much less of a
problem, and sharing should be *good* in that case.

And for the root disk we declare explicitly that installers may only
drop the root= param from the kernel cmdline if the OS is installed as
first root partition on the disk. Otherwise it *must* specify it to make
sure the right partition is found at boot.

Putting this altogether this should work fine for multi-boot cases. That
said, the benefit of not requiring an /etc/fstab is primarily one for
simpler "appliance" style disk images where only one OS is installed,
not for the super generic full distro cases that want to support every
possible storage setup. For example, I wouldn't expect anaconda to ever
drop the root= param from the kernel cmdline of its installations. I
mean, given that anaconda wants to support booting LVM on LUKS on RAID
and iscsi on bonded vlans and whatever else, there's no auto-discovery
like this possible anyway, and hence they can just keep the root= in
there for everybody...

> FAQ #1 talks about /usr and /etc, but /etc is almost always
> in the root partition, isn't it?

Well, sure it is, and so is /var... But it certainly makes sense to have
a seperate /etc in some cases. For example, in virtualized environments
it might make sense to share a single fixed read-only /usr between a
large number of containers that each have a private /etc and /var. 

I mean, this is the FAQ section, I am not saying whether splitting these
things makes sense or no sense at all, I am just saying that
auto-discovery makes no sense for these cases...

Lennart

-- 
Lennart Poettering, Red Hat