# Rip recovery — three-layer architecture
`libfreemkv` supports a multi-stage rip model for damaged or protection-bearing
discs: a fast forward sweep that tolerates read failures, in-loop request-size
adaptation that survives transient drive trouble without bailing, and targeted
retry passes against a persistent bad-range map. The stream pipeline
(`DiscStream` + `input`/`output`) operates against the resulting ISO image, so
the mux stage never touches the drive.
Recovery is layered cleanly. Each layer has one responsibility and does not
reach into the others.
| 1 — Bad-range retry | `Disc::patch` (multi-pass over the mapfile) | Re-reads non-`+` ranges with the long timeout. Idempotent; call N times. |
| 2 — Single-shot primitive | `Drive::read` in `src/drive/mod.rs` | One CDB, one timeout, one result. No inline retries, no SCSI reset. |
| 3 — In-loop request adaptation | `DiscStream::fill_extents` adaptive batch sizer | Halves the batch on failure, retries at the same LBA, walks back up on a clean-read streak. |
The caller orchestrates layer 1. Autorip's `rip_disc` loops `copy` then
N × `patch` per the `MAX_RETRIES` config, then hands the ISO off to the
existing mux pipeline. Layer 3 runs inside any consumer of `DiscStream`
(direct PES pipeline, ISO playback, etc.) without caller involvement.
Three primitives compose the disc-side flow:
| `Disc::copy` | disc → ISO. Writes a sidecar `.mapfile`. Opt-in skip-forward on failure. |
| `Disc::patch` | Re-reads bad ranges from the drive. Idempotent; call N times. |
| `DiscStream` (ISO source) | Reads sectors from the ISO, feeds decrypt → demux → codec → mux. |
## Data model
### Mapfile
Format: [ddrescue](https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html)-compatible
plain text, greppable, tool-interoperable. Flushed to disk on every `record()`
so a crashed rip loses at most one block.
```
# Rescue Logfile. Created by libfreemkv v0.13.6
# Current pos / status / pass / pass_time
0x000000000 ? 1 0
# pos size status
0x000000000 0x12a35d000 +
0x12a35d000 0x000003000 -
0x12a360000 0x009c4a000 +
0x12d00a000 0x000064000 *
```
Status characters match ddrescue:
| `?` | Not yet attempted |
| `*` | Fast-pass failed; needs edge-trim |
| `/` | Trimmed; interior needs sector scrape |
| `-` | Unreadable this session |
| `+` | Finished (good) |
Position and size are hex byte offsets into the ISO.
### `CopyOptions` and `PatchOptions`
Defaults preserve pre-`0.11.21` behavior — abort on the first unreadable
sector. Opt in to the recovery-friendly path:
```rust
CopyOptions {
skip_on_error: true, // zero-fill bad blocks, continue
skip_forward: true, // exponential skip-forward after a failure
resume: true, // pick up from an existing ISO + mapfile
decrypt: false, // keep the ISO a raw disc image
..Default::default()
}
```
## Algorithm
### Pass 1 — fast sweep (`Disc::copy`)
1. Read 64 KB (32 sectors, one BD ECC block) at the current LBA via
`Drive::read(.., recovery=false)` — short 1.5 s timeout, single shot.
2. On success: mark the range `+`, advance by one block.
3. On failure (with `skip_on_error`): zero-fill the block in the ISO, mark
it `*`, advance.
4. If `skip_forward` is set: after a failure, jump ahead by an exponentially
growing amount (256 KB initial, doubling on consecutive failures, capped at
1% of disc). The skipped bytes are also marked `*` — `patch` will visit
them later.
5. Reset the skip size to 256 KB on the first success after a failure.
Pass 1 completes when every byte has terminal status (`+`, `-`, or the caller
bails via the halt flag).
### Pass 2+ — patch (`Disc::patch`)
`Disc::patch` reads the mapfile and iterates every non-`+` range. For each:
1. Issue a drive read via `Drive::read(.., recovery=true)` — long 30 s
timeout, still single shot. Drive firmware does its own ECC and retries
inside that window; userspace does not pile on additional retries here.
2. On success: write the good bytes into the ISO at the exact byte offset,
mark `+`.
3. On failure: mark `-`.
4. Update the mapfile after every block — crash-safe resume.
Idempotent. Call `patch` N times for N retry attempts; typically the caller
stops early if a pass recovers zero bytes (structure-protected sectors will
never yield).
### In-stream — adaptive batch halving (`DiscStream::fill_extents`)
When a consumer reads a `DiscStream` directly (no ISO intermediate),
`fill_extents` runs an adaptive sizer in front of `Drive::read`:
1. Try the current preferred batch size (e.g. 32 sectors, one BD ECC block).
2. On failure: halve the batch and retry at the same LBA. Emit
`EventKind::BatchSizeChanged { reason: Shrunk }`.
3. On a clean-read streak: probe back up toward the preferred size. Emit
`EventKind::BatchSizeChanged { reason: Probed }`.
4. If a single-sector read fails: skip (zero-fill, emit
`EventKind::SectorSkipped`) when `skip_errors` is set, otherwise return
`Err(DiscRead)`.
This is layer 3. It exists so a transient single-sector glitch in a 32-sector
batch can be isolated and read individually without the caller needing to
implement retry logic.
## Design choices
**`Drive::read` is single-shot.** No inline retry phases, no SCSI reset,
no eject cycle. The `recovery` flag controls only the per-CDB timeout
(1.5 s vs. 30 s); on any failure it returns `Err(DiscRead)` immediately.
Inline recovery (5× gentle retry → close + SCSI reset + reopen → 5× more)
was removed in 0.13.6. See `freemkv-private/postmortems/2026-04-25-stop-wedge-and-zero-kbs.md`
for rationale: the inline reset on the LG BU40N (Initio USB-SATA bridge)
wedged drive firmware below the bridge without ever recovering a sector,
and the gentle-retry phase produced long stretches of 0 KB/s with no
recoveries to show for it. Recovery responsibility is now layered: layer 1
handles ranges, layer 3 handles request size, neither touches the
wedge-prone reset path.
**No `MODE SELECT` to disable drive retries.** Research showed neither ddrescue
nor MakeMKV does this. Drive firmware has access to raw analog signal, laser
power control, and drive-specific ECC tuning that userspace can't replicate —
disabling it throws away recovery headroom on marginal sectors. We fail fast
via short SG_IO timeouts in pass 1 and let the firmware work the long timeout
in pass 2 / patch.
**No SCSI reset from any retry path.** `SgIoTransport::reset` (Linux) is
trimmed to a kernel SG_IO state flush plus ALLOW MEDIUM REMOVAL — the
`SG_SCSI_RESET` ioctl and STOP/START UNIT escalation were removed in 0.13.6.
The macOS reset (which had been a no-op) was removed entirely. The top-level
`scsi::reset()` / `reset_with_timeout()` / `reset_blocking()` wrappers were
also removed (no callers). The remaining `Drive::reset()` is only invoked
explicitly by callers that need an eject-cycle escape hatch — it is never
reached from a read path.
**ISO intermediate, even for single-pass.** Pass 1 always writes an ISO. The
mux stage reads the ISO via `IsoSectorReader`. For single-pass (no retries),
this adds ~2-3 min (local disk mux) but gains resumability across crashes,
re-muxability without re-ripping, and a persistent forensic artifact. Callers
who need pure speed can bypass and use `DiscStream::new(Box::new(drive), …)`
directly — the lib doesn't forbid it, and layer 3 (adaptive batch halving)
still applies there.
**Mapfile in ddrescue format.** Plain text so users can `less` it, `diff` it,
or feed it to ddrescue's own tooling. Crash-safe (flush-per-record). Entries
coalesce on adjacent same-status ranges so files stay small.
**Patches target `-`, `*`, `/`, and `?` alike.** The status state machine is
ddrescue's but `patch` collapses the distinction — it just tries every
non-finished range with the long timeout. Future work can specialize (trim vs.
scrape vs. retry with direction reversal) if there's measured benefit.
## References
- [ddrescue manual, Algorithm chapter](https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html)
- [ddrescue optical media notes](https://www.electric-spoon.com/doc/gddrescue/html/Optical-media.html)
- Source: [`src/disc/mapfile.rs`](../src/disc/mapfile.rs), [`src/disc/mod.rs`](../src/disc/mod.rs) (`Disc::copy`, `Disc::patch`), [`src/drive/mod.rs`](../src/drive/mod.rs) (`Drive::read`), [`src/mux/disc.rs`](../src/mux/disc.rs) (`DiscStream::fill_extents`).