EntropyFS

de Beer, R. (2026). EntropyFS: Entropy-Native Configurational Storage as a Filesystem Substrate - Broad Prior-Art Technical Disclosure and Research Architecture (Version v1.0). Zenodo. https://doi.org/10.5281/zenodo.22092869

Persist irreducible state. Materialize structure. Preserve exact bytes. Measure everything.

EntropyFS is a native-Rust, mountable Linux filesystem whose central research premise is:

Logical bytes are an interface presented to applications. They do not have to be the primary persisted representation. EntropyFS persists the minimum exact reversible entropy/configuration state necessary to reproduce those bytes.

The defining equation:

X = Materialize(D)

X is the exact logical byte sequence; D is the persisted representation descriptor. A more general family:

X = T(E(U, S, P)) ⊕ R

U = versioned entropy universe · S = seed/state · P = rank/coordinate · T = bounded reversible transform · R = exact residual · E = deterministic materialization.

EntropyFS does not claim to evade information theory: the SSD still stores physical bits. Its innovation is to make entropy state, mathematical rank/configuration, immutable references, deterministic generators, transforms, and irreducible residuals first-class storage representations instead of assuming logical byte blocks must themselves exist at rest. For random/encrypted/incompressible data it converges gracefully toward ordinary physical storage (RAW fallback) — that is a success condition, not a failure.

Current status

Measured results

All performance and storage-density claims are governed by docs/performance/methodology.md; admitted results live in evidence/performance/ (see evidence/performance/INDEX.md).

FUSE-frontend before/after pair (Phase 6, same workloads, same machine):

The fsync regression is the measured cost of deferred durability and is reported honestly; the before half reproduces the crashes the oversized-descriptor fix (Phase 6) eliminated.

What EntropyFS does and does not claim:

• The synthetic ablation fixture (evidence/ablation-2026-08-25.json) is an ablation fixture, never a headline: on a corpus containing four unique 64 KiB chunks its 16.876× is dominated by content-addressed dedup. The campaign's structured-corpus ratios (up to 1,328×) are structural + EXACT_REF aliasing + CAS object sharing — attribution is now measured, not labeled: per-run accounting separates cas_shared_bytes_saved (a store invariant) from exact_ref_bytes_saved (the gated alias representation), and A1 is pure byte rANS with SequenceRans as the post-registration E1 step (campaign-1787671040-923df7b/). The earlier “dedup = 0” and “dedup-dominated” statements conflated the two layers and are amended in evidence/performance/INDEX.md, never rewritten.
• The campaign's ablation evidence is two tables, both kept forever: the strict cumulative ladder A0–A8 (each step adds one mechanism) and the leave-one-out table (one mechanism disabled at a time). The first campaign's nine-row table is the leave-one-out table; it predates the two-table rule and is amended as such in evidence/performance/INDEX.md (protocol note, never rewritten).
• The campaign's DSFB investigation is a sealed three-era series, all with byte-identical physical representations: RANS-era 765.4 vs 334.7 MiB/s (2.29×, user CPU halved — 67d977a), SequenceRans-era 773.9 vs 717.1 MiB/s (~8%, b165d60), and CAS-era 1,120.8 vs 1,106.1 MiB/s (~1.3%, 923df7b). DSFB's marginal benefit collapsed as the SequenceRans floor simplified the search landscape — evidence for its assigned role as candidate-search budget intelligence, not compression; its marginal value now is small on this tiny synthetic corpus and its proper counters are deferred. Historical numbers are preserved in evidence/performance/INDEX.md; the 2.29× is not a current claim.
• The source-corpus progression is now sealed across eras: 923df7b pure byte rANS 1.633× / standalone SequenceRans 3.556× (with zstd-per-64KiB -1 at 3.739× — the per-extent floor was within 5%, and the gap to whole-file zstd was cross-chunk context), 8250f6b EntropyFS full 4.070× with SequenceDict on the packed stream, and the Phase-9C/9D/9E tree court answering the open mount-level question: on a real tree of separately-written files, per-file zstd -1 is ~3.5× and EntropyFS per-file writes are 2.194×, rising to 2.354× post-GC after the shared-dict pool + deep background pass — so most of the packed-stream density was indeed cross-FILE structure, and the shared dictionary (plus the deep matcher, which lifts the standalone src-pack floor from 3.736× to 3.786×) recovers a measured part of it. The Phase-9F gap decomposition (sealed in the 9F tree court) shows where the remaining gap lives: the anchor policy is NOT the cap (zstd with the same per-directory anchor gains only ~8.5%, less than EntropyFS's pool already recovers), and the residual gap is ~2/3 per-extent persistence overhead (per-chunk multi-stream rANS models + descriptors on small files; ~26.5% of the EntropyFS footprint) and ~1/3 coder quality. Recorded as current state, not claims.
• Phase-9G0 (sealed 80e36c8) validated the 9F diagnosis directly: the per-stream RAW/rANS gate now includes the persisted model bytes (was: rANS whenever enc < raw, which persisted models that could never pay for themselves), cutting the sequence families' model objects on the real tree from 277.6 KB to 74.3 KB — per-extent overhead 26.5% → 11.1% of footprint, tree court 2.388× → 2.775× post shared-dict, src corpus 4.327×. The model-sharing oracle (diagnostic) then decided the 9G design: sharing one model across an extent's streams is falsified (−125 KB); one aggregate model per stream type per directory cohort is validated (+49.6 KB) and needs no format change (the model object is content-addressed and CAS-amortized); bundle pools of 2/4 lose to the single aggregate. 9G0's sealed campaign numbers are in evidence/performance/INDEX.md; the oracle is a diagnostic, not a claim.
• Phase-9G (sealed 60ecaf2) implemented the oracle's S2: the background model_bundle_pass trains one aggregate model per stream type per directory cohort and re-encodes members against it (per-stream RAW fallback), rewriting only when the cohort's total strictly falls. No format change and no new feature bit — model objects were already content-addressed descriptors. Tree court: shared-dict 2.813× → 2.881× (1,065,145 B reachable, 65 rewrites; the real post-GC saving 25.7 KiB exceeds the 7.5 KiB conservative cohort-accounted claim because GC reclaims the superseded per-extent models). The win is better-trained aggregate models (the enc side), not model dedup — the model-bytes metric is flat, recorded as-is.
• Phase-9H (sealed 0a03ece) — physical convergence: the 9G tree court exposed that the optimized tree occupied 3.66 MB of backing for 1.07 MB reachable (0.84× logical). The physical scanner (store/physical.rs) reconciled every segment byte and falsified the index-hidden-duplicate hypothesis on that workload (index-hidden = 0): the dead bytes were 2.66 MB of BtreeNode records staged by the GC chunk-index REBUILD, which rebuilt the tree with repeated COW inserts — every intermediate path version written and indexed. Fixes: GC victim selection now uses scanned physical occupancy; the rebuild bulk-loads the tree bottom-up (each final node staged exactly once); entropyfs gc --compact (compact_full) converges the backing and is idempotent. Tree court: backing 9,129,988 B → 1,100,161 B; post-GC reconciliation = reachable 1,100,157 B + 0 B dead + 0 B index-hidden + 0 B unindexed + 4 B format; full compaction = reachable
  • 4 B (0.00% of logical); second compaction reclaims 0. The 2.88× representation win is now a real ~2.9× capacity win — the physical floor no longer eats the representation state.
• The campaign's H2 experiment (synthetic drift corpus) is now a sealed controlled series: 67d977a +7.2% (RANS-era floor), a6641d1 −24% (SequenceRans floor, positional residuals only), 43bf17e +35.2% (SequenceRans floor + BASE_SEQUENCE shift-aware deltas), and 923df7b +40.6% (marginal costing). The shuffled control grows in the delta campaign because copy/literal deltas also exploit structural similarity between unrelated-history chunks — the control no longer isolates pure temporal causality, and that confounding is itself recorded as the finding.
• The competitive filesystem court is a sealed zero-waiver series at 9H: the density comparison is now computed and sealed by the tooling itself (fs-court-1787688843-b4abc71/, privileged docker VM, symmetric buffered/durable/warm/cold rules) — the same corpus apparent sum (136,162,907 B) over each filesystem's complete state (whole loop image allocated for XFS/Btrfs, complete store backing for EntropyFS; both include filesystem metadata; derivation footnoted in report.md). EntropyFS is reported in two states — foreground 1.825× (74.6 MB post-GC) and settled 1.994× (68.27 MB) after background optimize + full compaction (5.42 s, 1.047× physical write amplification) — and the settled density beats Btrfs+zstd 1.724× (79.00 MB image) on the same corpus set (64 MiB incompressible random + 64 MiB zeros + src + tgz); Btrfs raw 0.941×, XFS 0.668× (metadata exceeds the corpus). The throughput gaps are recorded honestly: src tiny-file writes 10.2 MiB/s vs 100–457, 64 MiB random writes 68 vs ~5,900 MiB/s. The write path is now the dominant weakness; the court points the next phase at performance (Phase 10), not another entropy codec. (The earlier courts' hand-derived 1.65×/1.72× Btrfs figures are amended in evidence/performance/INDEX.md, never rewritten.)
• Random/encrypted/already-compressed data falls back toward RAW (urandom 0.997×, zstd -19 pack 0.993×) — the honest negative control.

Honesty rules

• A 128-bit seed does not "store" a gigabyte. Descriptor bits select at most 2^k states; every persisted bit is accounted (docs/theory/information-accounting.md).
• No hidden corpus, no network, no RNG in materialization, no CPU-dependent floating point. The universe specification is part of the format version.
• No arbitrary generator programs: the descriptor language is bounded and not Turing-complete (docs/adr/0005-representation-set.md).
• DSFB has zero decoding authority (docs/adr/0004-dsfb-observer.md).
• statfs reports physical capacity; effective ratio is an observation, never a promise (docs/adr/0018-statfs.md).
• Every optimization claim requires reproducible evidence (docs/performance/methodology.md).

Building

rustup toolchain install stable --profile minimal --component rustfmt,clippy
cargo build --release
cargo test
cargo clippy --all-targets -- -D warnings
cargo fmt --check

Target platform

CachyOS/Arch Linux, x86-64, FUSE (/dev/fuse + fusermount3; kernel CONFIG_FUSE_FS=y). No custom kernel, no out-of-tree module, no reboot.

Reading order

• docs/architecture/overview.md — architecture map
• docs/theory/entropy-medium.md — the information-theory boundary statement
• docs/format/ondisk-v1.md — the on-disk format
• docs/recovery/crash-consistency.md — the crash protocol
• docs/adr/ — all architecture decision records

License

MIT OR Apache-2.0, at your option.