Deterministic output

The default materialization is byte-reproducible: the same source and the same inputs produce a byte-identical image on any machine.

Every value a filesystem would normally draw from its environment is an input instead. The two such values are:

• the filesystem UUID is a 16-byte value supplied by the caller;
• timestamps are supplied by the caller.

A caller that wants clock- or random-derived values computes them and passes them in explicitly, keeping that choice out of the default path.

Inode-number assignment is a fixed function of the source, in sorted path order, so two runs over the same source assign identical inode numbers — and therefore write identical bytes. Anything that reaches on-disk order (directory entries, block groups) is sorted or otherwise fixed into a determinate order.