Comment # 3 on bug 967148 from Andreas Stieger

(In reply to ill lume from comment #2)
> Snapper causing upgrades to fail is a bug.

No, you already explained what happens when you upgrade: The disk fills up.
Because this is what you asked snapper to do. If you do not have sufficient
disk space to perform the upgrade and keep the COW snapshots, disable the
functionality.

Beyond that, your reports lacks information required to classify this as a
defect.