Comment 17 for bug 1974100

I haven't come across this before. Our builds for the cloud images from cloud-images.ubuntu.com have the following cleanup code run that should be preventing this sort of thing:

https://git.launchpad.net/livecd-rootfs/tree/live-build/functions#n231

hard to read exact ramifications, but the gist is we create a filesystem, mount it, create the image, then at the end during clean up we run

# rootfs_dev_mapper is set in https://git.launchpad.net/livecd-rootfs/tree/live-build/functions#n62 when mounting an image

e2fsck -y -E discard ${rootfs_dev_mapper}
zerofree ${rootfs_dev_mapper}

before running kpartx to remove the mount

this _should_ be discarding the empty blocks. That matches the expectation b1 and b2.

But I haven't seen

> The cloudimage extended and used as root grows more lost blocks (seems like 5 MB above)

so that bears investigating.