[Bug 1905990] [NEW] UC20 amd64 images are unnecessarily big

[Łukasz Zemczak]

Public bug reported:

[Impact]

To work-around a bug in ubuntu-image, we hard-coded a `--image-size 8G` in livecd-rootfs for UC20 image builds. The reason for that was that ubuntu-image, in its initial UC20 image build implementation, was skipping yet-to-be-created partitions while calculating the final image size. This was fixed in ubuntu-image 1.10, and now the workaround is no longer required.
Additionally, the HWE team asked if it would be possible for the amd64 images to be smaller than 8GB as they need them to fit on 4GB storage. Without the hard-coded image size the UC20 images are around 3.1GB in size - which would be perfect for their needs.

[Test case]

Build a -proposed based UC20 focal amd64 image via live-build. Download
the image, decompress it and run `parted <imagefile>`. Print the
partition table and make sure that the image is now big enough for all
the contents (so at least 3GB) but smaller than the previous hard-coded
size (8GB).

[What could go wrong]

The only parts I could see regressing is the thing we want to actually
fix: so ultimately ubuntu-image not calculating the size properly. But
this was already verified before pushing this SRU. The change only
affects ubuntu-core focal+ builds, so the regression risk is really
minimal.

** Affects: livecd-rootfs (Ubuntu)
     Importance: Medium
     Assignee: Łukasz Zemczak (sil2100)
         Status: In Progress

** Affects: livecd-rootfs (Ubuntu Focal)
     Importance: Undecided
         Status: In Progress

** Affects: livecd-rootfs (Ubuntu Groovy)
     Importance: Undecided
         Status: Invalid

** Also affects: livecd-rootfs (Ubuntu Groovy)
   Importance: Undecided
       Status: New

** Also affects: livecd-rootfs (Ubuntu Focal)
   Importance: Undecided
       Status: New

** Changed in: livecd-rootfs (Ubuntu Groovy)
       Status: New => Invalid

** Changed in: livecd-rootfs (Ubuntu Focal)
       Status: New => In Progress

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https://bugs.launchpad.net/bugs/1905990

Title:
  UC20 amd64 images are unnecessarily big

Status in livecd-rootfs package in Ubuntu:
  In Progress
Status in livecd-rootfs source package in Focal:
  In Progress
Status in livecd-rootfs source package in Groovy:
  Invalid

Bug description:
  [Impact]

  To work-around a bug in ubuntu-image, we hard-coded a `--image-size 8G` in livecd-rootfs for UC20 image builds. The reason for that was that ubuntu-image, in its initial UC20 image build implementation, was skipping yet-to-be-created partitions while calculating the final image size. This was fixed in ubuntu-image 1.10, and now the workaround is no longer required.
  Additionally, the HWE team asked if it would be possible for the amd64 images to be smaller than 8GB as they need them to fit on 4GB storage. Without the hard-coded image size the UC20 images are around 3.1GB in size - which would be perfect for their needs.

  [Test case]

  Build a -proposed based UC20 focal amd64 image via live-build.
  Download the image, decompress it and run `parted <imagefile>`. Print
  the partition table and make sure that the image is now big enough for
  all the contents (so at least 3GB) but smaller than the previous hard-
  coded size (8GB).

  [What could go wrong]

  The only parts I could see regressing is the thing we want to actually
  fix: so ultimately ubuntu-image not calculating the size properly. But
  this was already verified before pushing this SRU. The change only
  affects ubuntu-core focal+ builds, so the regression risk is really
  minimal.

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