[OE-core] Contents of non-rootfs partitions

[Maciej Borzęcki]

On Wed, Nov 23, 2016 at 9:40 AM, Enrico Joerns <ejo at pengutronix.de> wrote:
> On 11/22/2016 12:54 PM, Kristian Amlie wrote:
>>
>> On 22/11/16 12:10, Patrick Ohly wrote:
>>>>
>>>> ...
>>>
>>>
>>> All of these introduce some special mechanism. Let me propose something
>>> that might integrate better with the existing tooling:
>>>
>>> The "rootfs" directory gets redefined as representing the entire virtual
>>> file system. When creating a disk image, it gets split up into different
>>> partitions based on the image configuration.
>>>
>>> For example, the /home or /data directories in the rootfs could hold the
>>> content that in some image configurations goes into separate partitions.
>>>
>>> The advantage of this approach is that the tooling for staging content
>>> for image creation does not need to be changed. The same staged content
>>> then can be used to create different images, potentially even using
>>> different partition layouts.
>>
>>
>> That's a very good idea. I think it beats all of my suggestions!
>
>
> I totally agree with this solution, because it is the same approach we are
> already using for several years.
>
> Instead of wic we are using the genimage tool
> (http://public.pengutronix.de/software/genimage/) together with a Yocto
> class for easing image creation, but its pretty much the same concerning its
> base purpose.
>
> We called the option to split up 'mountpoint' while assuming that the
> splitted partition will be mounted to the same location in the file system
> it was taken from.
>
> An example configuration would look like this (stripped down to the
> essential parts):
>
>   image my-disk-image {
>           [...]
>           partition root {
>                   image = "rootfs.ext4"
>                   [...]
>           }
>
>           partition home {
>                   image = "home.ext4"
>                   [...]
>           }
>   }
>
>   image rootfs.ext4 {
>           [...]
>           mountpoint = "/"
>   }
>
>   image home.ext4 {
>           [...]
>           mountpoint = "/home"
>   }
>
> Maybe a similar approach could be used in wic, too.

I went through the README from genimage repo
https://git.pengutronix.de/cgit/genimage/tree/README. Definitely looks
interesting and covers storage medias that are not supported by wic
right now. The split option does not seem to be documented though. If I
understood you correctly, the partition is seeded with data from
location specified by `mountpoint` inside IMAGE_ROOTFS. Is that correct?

I see there's a large overlap in functionality with wic (at least for
block devices), so perhaps the best way would be introduce genimage as
another IMAGE_CMD.

I've quickly checked at https://layers.openembedded.org and there are no
recipes for genimage/libconfuse listed there. Do happen to have done
integration with OE in some private repo already? Would it be possible
for you to post the relevant patches?

-- 
Maciej Borzecki
RnDity