[j16sdiz]

>The above fails in only one case: Free space on the drive is very low.

No. Most of the block will be marked as unsure in first step -- because most of them had been used before thanks to CoW

[londons_explore]

A heuristic could be written like 'protect the latest version of each node, plus 2 prior versions, but anything older you find, treat it as free apace'.