Deterministic output

The default materialization is byte-reproducible in every family. 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. Which values those are is a property of the format, so each family names its own. The list is short in all four, because a format records only what it has a field for:

• ext2, ext3, and ext4 take three: the filesystem UUID, a 16-byte value, the directory-hash seed that a hash-indexed directory orders its names by, and timestamps. The image records the hash seed beside the byte signedness names are compared under. A directory built on one host therefore reads the same way on another.
• FAT12, FAT16, and FAT32 take two: the volume serial number, a 32-bit value the boot sector and its backup carry, and timestamps. A directory entry’s dates are civil dates rather than instants, and the conversion is UTC. Nothing about the machine’s own zone reaches an image either.
• exFAT takes one: the volume serial number. An empty exFAT volume records no time anywhere, so timestamps reach an image only with the files that carry them. An empty volume is reproducible from that one value alone.
• btrfs takes five: the filesystem id a person sees, the chunk tree id every tree block repeats, and the device id. The other two are the top-level subvolume id and one instant. The format’s own tooling draws all five from a random source and the clock. Making them inputs is what makes a filesystem’s bytes a function of what it was asked for.

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

What a filesystem assigns internally is fixed the same way. Inode-number assignment is a function of the source in sorted path order, and so is cluster allocation. Two runs over the same source therefore place the same objects in the same places, and write identical bytes. Anything that reaches on-disk order (directory entries, block groups, an entry set’s position in its directory) is sorted or otherwise fixed into a determinate order.

The scope of the guarantee

ferrosys guarantees identical bytes for one version, one feature set, and identical inputs. Across versions the guarantee holds where the on-disk feature set is pinned exactly. ext::FeatureSet::DEFAULT is frozen and safe to pin, and ext::FeatureSet::LATEST tracks current mke2fs and moves between releases by design.

A consumer pinning an on-disk contract records the whole resolved feature set, which ext::FeatureSet::pin emits. A feature-name list is not enough, because it omits the block and inode sizes.