Re: [RFC PATCH] Add new partition type API and convert GPT type checks.

[Michael Marineau]

On Mon, Sep 22, 2014 at 4:06 PM, Vladimir 'φ-coder/phcoder' Serbinenko
<address@hidden> wrote:
> On 22.09.2014 22:03, Michael Marineau wrote:
>> that is what I need to make use of and copying the code to re-read the
>> type field yet again felt silly. My final goal is actually a module
>> that will search for two partitions of the same type and select
>> between them based on some other metadata. Since searching by
>> partition type is already a common task in the grub code base
>> providing an API to do so seemed like a good starting point. Others
>> will likely find extending this to support label and uuid more useful.
>> For example with GPT the Linux kernel can use
>> "root=PARTUUID=<someuuid>" directly, unlike LABEL= or UUID= which must
>> be implemented in an initrd.
> Using partition type has proven to be unrelaiable in the past and grub2
> uses as little of it as possible. Why can't your goals be achieved with
> FS UUIDs?

I'm working on CoreOS which updates /usr via read-only filesystem
images, switching between using two different partitions in an A/B
setup. The UUID in the filesystem is set when the image is created and
has nothing to do with which of the two coppies of the filesystem I
want to boot. That information is instead in the partition table, both
/usr partitions have the same special type and a set of flags that
determine which of the two to try next. It is based on the scheme
ChromeOS uses except we only have /usr instead of root and kernel
partitions.

http://www.chromium.org/chromium-os/chromiumos-design-docs/disk-format#TOC-Selecting-the-kernel

Right now we implement this logic in an initrd and launch the final
kernel (which must match the /usr partition) via kexec. Instead I'm
moving that into a grub module because kexec does not work on many
platforms.