matthew
42e3dd3240
- README quick-start gains a multi-pass example using the new Disc::copy + Disc::patch primitives. - New docs/rip-recovery.md documents the two-stage rip model: mapfile format (ddrescue-compatible), CopyOptions/PatchOptions surface, the pass-1/pass-2 algorithm, and the design decisions (why no MODE SELECT, why ISO intermediate, why ddrescue mapfile). No code change.
128 lines
5.5 KiB
Markdown
128 lines
5.5 KiB
Markdown
# Rip recovery — multi-pass architecture
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`libfreemkv` supports a two-stage rip model for damaged or protection-bearing
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discs: a fast forward sweep that tolerates read failures, followed by targeted
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retry passes against a persistent bad-range map. The stream pipeline
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(`DiscStream` + `input`/`output`) operates against the resulting ISO image, so
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the mux stage never touches the drive.
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Three primitives compose the flow:
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| Primitive | What it does |
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|---------------------------|-----------------------------------------------------------------------|
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| `Disc::copy` | disc → ISO. Writes a sidecar `.mapfile`. Opt-in skip-forward on failure. |
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| `Disc::patch` | Re-reads bad ranges from the drive. Idempotent; call N times. |
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| `DiscStream` (ISO source) | Reads sectors from the ISO, feeds decrypt → demux → codec → mux. |
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The caller orchestrates. Autorip's `rip_disc` loops `copy` then N × `patch` per
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the `MAX_RETRIES` config, then hands the ISO off to the existing mux pipeline.
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## Data model
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### Mapfile
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Format: [ddrescue](https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html)-compatible
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plain text, greppable, tool-interoperable. Flushed to disk on every `record()`
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so a crashed rip loses at most one block.
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```
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# Rescue Logfile. Created by libfreemkv v0.11.22
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# Current pos / status / pass / pass_time
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0x000000000 ? 1 0
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# pos size status
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0x000000000 0x12a35d000 +
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0x12a35d000 0x000003000 -
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0x12a360000 0x009c4a000 +
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0x12d00a000 0x000064000 *
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```
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Status characters match ddrescue:
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| Char | Meaning |
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|------|----------------------------------------------------|
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| `?` | Not yet attempted |
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| `*` | Fast-pass failed; needs edge-trim |
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| `/` | Trimmed; interior needs sector scrape |
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| `-` | Unreadable this session |
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| `+` | Finished (good) |
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Position and size are hex byte offsets into the ISO.
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### `CopyOptions` and `PatchOptions`
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Defaults preserve pre-`0.11.21` behavior — full drive recovery on every read,
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abort on the first unreadable sector. Opt in to the recovery-friendly path:
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```rust
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CopyOptions {
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skip_on_error: true, // zero-fill bad blocks, continue
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skip_forward: true, // exponential skip-forward after a failure
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resume: true, // pick up from an existing ISO + mapfile
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decrypt: false, // keep the ISO a raw disc image
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..Default::default()
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}
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```
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## Algorithm
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### Pass 1 — fast sweep
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1. Read 64 KB (32 sectors, one BD ECC block) at the current LBA.
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2. On success: mark the range `+`, advance by one block.
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3. On failure (with `skip_on_error`): zero-fill the block in the ISO, mark
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it `*`, advance.
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4. If `skip_forward` is set: after a failure, jump ahead by an exponentially
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growing amount (256 KB initial, doubling on consecutive failures, capped at
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1% of disc). The skipped bytes are also marked `*` — `patch` will visit
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them later.
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5. Reset the skip size to 256 KB on the first success after a failure.
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Pass 1 completes when every byte has terminal status (`+`, `-`, or the caller
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bails via the halt flag).
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### Pass 2+ — patch
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`Disc::patch` reads the mapfile and iterates every non-`+` range. For each:
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1. Issue a drive read with full recovery enabled (SCSI-level retries,
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ECC recovery, the lot).
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2. On success: write the good bytes into the ISO at the exact byte offset,
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mark `+`.
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3. On failure: mark `-`.
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4. Update the mapfile after every block — crash-safe resume.
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Idempotent. Call `patch` N times for N retry attempts; typically the caller
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stops early if a pass recovers zero bytes (structure-protected sectors will
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never yield).
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## Design choices
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**No `MODE SELECT` to disable drive retries.** Research showed neither ddrescue
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nor MakeMKV does this. Drive firmware has access to raw analog signal, laser
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power control, and drive-specific ECC tuning that userspace can't replicate —
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disabling it throws away recovery headroom on marginal sectors. We fail fast
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via short SG_IO timeouts instead, and we avoid per-sector probing on first
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contact by using large blocks + skip-forward.
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**ISO intermediate, even for single-pass.** Pass 1 always writes an ISO. The
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mux stage reads the ISO via `IsoSectorReader`. For single-pass (no retries),
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this adds ~2-3 min (local disk mux) but gains resumability across crashes,
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re-muxability without re-ripping, and a persistent forensic artifact. Callers
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who need pure speed can bypass and use `DiscStream::new(Box::new(drive), …)`
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directly — the lib doesn't forbid it.
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**Mapfile in ddrescue format.** Plain text so users can `less` it, `diff` it,
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or feed it to ddrescue's own tooling. Crash-safe (flush-per-record). Entries
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coalesce on adjacent same-status ranges so files stay small.
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**Patches target `-`, `*`, `/`, and `?` alike.** The status state machine is
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ddrescue's but `patch` collapses the distinction — it just tries every
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non-finished range with full recovery. Future work can specialize (trim vs.
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scrape vs. retry with direction reversal) if there's measured benefit.
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## References
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- [ddrescue manual, Algorithm chapter](https://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html)
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- [ddrescue optical media notes](https://www.electric-spoon.com/doc/gddrescue/html/Optical-media.html)
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- Source: [`src/disc/mapfile.rs`](../src/disc/mapfile.rs), [`src/disc/mod.rs`](../src/disc/mod.rs) (`Disc::copy`, `Disc::patch`)
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