allow .img that is the size of ubuntu-seed

[Kyle Nitzsche]

In Ubuntu Core 20, the built .img file includes only the ubuntu-seed partition. In my case it is < 500MB. But the created .imgfile is much larger (for example 3.4G on pi with a small set of seeded snaps).

It takes a long time to dd the .img file to the pi. A customer has requested the ability to generate a much smaller .img file, perhaps only as big as the ubuntu-seed partition. This seems to make sense since on uc20, all the other partitions are created at first boot time.

(Currently the -i flag can be passed to try to inform u-i to create a smaller .img, but if the requested size is less than the sum of the partitions declared in the gadget.yaml file, it is ignored and with a warning.)

The request is to either always create the .img with the minimum size needed (probably the size of ubuntu-seed) or to allow overriding the default size with a smaller one the user sets with -i or similar.

[Kiley Hykawy (kileyhykawy-nureva)]

Just to put some additional context around the problem. The reference Raspberry Pi Ubuntu Core 20.04 image is 3,550,225 KB (looking at Windows Explorer). Using Balena Etcher to flash an RPi Compute Module 4 takes over 10 minutes to flash and then 5 minutes to verify. That is way too much time to flash a device in the factory. Four units per hour (unless you scale up with multiple programming stations of course :)) makes it hard to get to production quantities.

So getting this image down to a much smaller size is really needed for commercial products.

[Kyle Nitzsche (knitzsche)]

Has this bug been triaged? thanks.