# music-mirror Maintain a lossy MP3 mirror of a lossless music library. Walks a source library and reproduces it, path for path, as MP3 in a separate tree. Tags and cover art are carried across; sources that are already MP3 are copied rather than re-encoded; mirror files whose source has been deleted are removed. **The source library is never written to** — it is mounted read-only in the supplied compose file, and nothing in the code opens it for writing. The intended use is an iPod. Apple's Music app cannot read FLAC at all, so a converted copy has to exist somewhere; this keeps that copy next to the library on a NAS instead of on a laptop, and keeps it current without a human remembering to do anything. ## How it decides what to do | Situation | Action | | -------------------------------- | ---------------------------------------- | | No mirror file | encode | | Source modified since the mirror | re-encode (a Lidarr quality upgrade) | | Mirror up to date | skip | | Source is already MP3 | copy verbatim | | Source gone | delete the mirror file, prune empty dirs | Freshness is modification time: an encoded file is stamped with its source's mtime, so a file is stale exactly when the two differ. There is no database to fall out of step with the library, which matters when something else — Lidarr, in this case — is the thing that owns and reorganises it. ### What that means for Lidarr | Lidarr does this | The mirror does this | | --------------------------------------- | -------------------------------------------------------------------------------- | | Replaces a file with a better rip | Re-encodes in place. Same path in, same path out, so no duplicate | | Upgrades MP3 to FLAC | Both map to the same `.mp3` mirror path, so the old one is overwritten | | Renames a track, album or artist folder | Old path pruned, new path encoded. Correct, but it re-encodes rather than moving | | Deletes an album or artist | Every orphaned mirror file is deleted and the emptied directories go too | Pruning is driven by what the pass actually found, not by guessing source filenames from mirror ones: a `.FLAC` source would not be found by a search for `.flac`, and the mirror file would be deleted and rebuilt on alternate passes for ever. If two sources want the same mirror path — a `01 Song.flac` next to a leftover `01 Song.mp3`, which is what an interrupted upgrade leaves — the better format wins, ties break on path, and the loser is logged. Without that rule both encode to the same destination and every pass finds one of them stale. The mtime is read _before_ encoding rather than after. A file still being written when the pass reaches it would otherwise be stamped with its final mtime while holding truncated audio, and never be revisited. Both encodes and copies are written to a temporary file and renamed into place, so an interrupted run cannot leave a truncated MP3 that the next run mistakes for finished work. Copies need it as much as encodes do: the mtime comes across with the bytes, so a half-written copy would look current for ever. A lock file in the mirror root stops two passes overlapping. ### Permissions Everything written into the mirror is made group-readable, and its directories group-traversable, so the mirror can be read back by whatever serves it. Neither writer does that unaided: the temporary file an encode renames into place is created `0600` regardless of the umask, and a straight copy of an existing MP3 inherits the mode of a source file in a library this tool does not own. Directories are handled by clearing the owner and group read/execute bits from the process umask, once, at startup. Owner as well as group, because a umask carrying `0400` produces directories of mode `0300` — writable and enterable, unreadable to the very run that created them. The `other` bits are left where the umask puts them: whether the mirror is world-readable is a genuine policy question, and so is its ownership. Mirror files written before this existed are topped up on the next pass. Their mtimes are correct, so nothing else would revisit them — and they are not re-encoded, only chmod'ed. ## Usage ```sh music-mirror --source /music --mirror /music-mp3 # one pass music-mirror --source /music --mirror /music-mp3 --interval 6h # keep running music-mirror --source /music --mirror /music-mp3 --dry-run # report only music-mirror --source /music --mirror /music-mp3 --subdir "Artist/Album" ``` | Option | Environment variable | Default | Meaning | | ------------ | ----------------------- | --------- | ---------------------------------------------- | | `--source` | `MUSIC_MIRROR_SOURCE` | — | Root of the lossless library, read-only | | `--mirror` | `MUSIC_MIRROR_MIRROR` | — | Root of the MP3 mirror | | `--quality` | `MUSIC_MIRROR_QUALITY` | `V0` | LAME VBR level `V0`–`V9`, or kbps e.g. `256` | | `--jobs` | `MUSIC_MIRROR_JOBS` | CPU count | Concurrent encodes | | `--interval` | `MUSIC_MIRROR_INTERVAL` | unset | Repeat forever, e.g. `45m`, `6h`, `1d` | | `--subdir` | — | unset | Limit the pass to one directory; skips pruning | | `--fat32-safe` | `MUSIC_MIRROR_FAT32_SAFE` | off | Name files so a FAT32 device accepts them | | `--no-prune` | — | off | Keep mirror files whose source has gone | | `--dry-run` | — | off | Report what would change, write nothing | `--subdir` never prunes: a partial pass cannot tell an orphan from a file outside its own scope. ### Concurrency LAME is single-threaded — ffmpeg reports `Threading capabilities: none` for `libmp3lame` — so throughput comes entirely from running several encoders at once, one process per file. `--jobs` defaults to the CPUs the process may actually use, which inside a container means the `cpus:` allowance rather than the host's core count. Each pass logs the number it settled on. As a rough guide, a Zen 3 core encodes about 40–60× realtime at V0 depending on clock, so six cores clear roughly 250 hours of audio per hour of wall clock. The first full pass is the expensive one; after that only new and changed files are touched. Lower `MUSIC_MIRROR_JOBS` if you would rather the NAS stayed responsive than finished sooner. Requires `ffmpeg` and `ffprobe` on `PATH`. The container image provides both. ## Running it on TrueNAS Scale `compose.yaml` is a Custom App definition. Adjust the two host paths and the `user:` to match your pool, then add it as a custom app. The image is published to this Gitea's registry on every release: ``` code.emmathe.dev/lyrathorpe/music-mirror:latest ``` Tags are `latest`, the full version, and the truncated `major.minor` and `major` forms; builds that are not releases are published as `sha-`. Point the mirror at its own dataset rather than a directory inside the music dataset — it is derived data, so it wants its own snapshot policy, its own quota, and its own SMB share. The tool refuses to run with a mirror inside the source tree. New Lidarr imports are picked up on the next pass. With `MUSIC_MIRROR_INTERVAL` at `6h` that is the worst case; run `--subdir` by hand if you want an album immediately. ## Tools Host-side scripts under `tools/`, not part of the container image. `sync-to-ipod.sh` does a whole transfer: submits the scrobbler log, checks the mirror, rsyncs, syncs and unmounts. ```sh tools/sync-to-ipod.sh /mnt/tank/media/music-mp3 /media/IPOD/Music tools/sync-to-ipod.sh -n /mnt/tank/media/music-mp3 /media/IPOD/Music # dry run ``` The destination is where the artist folders should end up — normally a subdirectory such as `/media/IPOD/Music`, not the card root. A subdirectory is the better target: `--delete` is confined to it, and the device path budget is derived from it rather than configured, so the two cannot disagree. It refuses to start unless the destination is on a mounted FAT filesystem. That check is also what catches an unmounted device — `/media/IPOD/Music` then resolves to the host's own root filesystem, and this refuses to empty that. `--help` says all of it. It also excludes `/.rockbox`, the scrobbler logs and the various filesystem metadata directories from deletion — the mirror does not contain them, and without the exclusion a sync to the card root would remove the Rockbox install. Progress is a single line that rewrites itself: ``` [ 24%] 12,345/49,600 3.2 GiB/13.1 GiB 4.4 MiB/s ETA 38m12s King Gizzard / Petro… ``` The estimate comes from rsync's `%l`, which gives each file's size as it completes. Bytes done over time elapsed is the same arithmetic rsync would do, and needs nothing it does not already print. The rate is measured over a trailing thirty seconds rather than the whole run, so it follows a device that slows down instead of averaging the slowdown away — and it is suppressed entirely for the first two seconds, where the window is microseconds wide and would report gigabytes per second. rsync says nothing at all while it builds its file list, which on fifty thousand files is minutes of apparent hang, and its own `progress2` percentage is computed against a list it has not finished discovering. So the script renders its own. **The percentage and the estimate are opt-in, via `-P`.** They need a total, the total needs a counting pass, and that pass walks and compares both trees in full exactly as the transfer does. Measured on a real card: read from the source at 35 MB/s and write to the card at 21 MB/s, yet the sync crawled — because the traversal, not the data, was the cost, and it was being paid twice. Without `-P` the line still shows the running count, the rate and the album in flight; only the two figures that needed the second walk are missing. Piped to a log it prints a plain line every thirty seconds instead, with no carriage returns, and a summary at the end either way. ### The Rockbox database Point `MUSIC_MIRROR_DATABASE_TOOL` at Rockbox's host-side builder and the sync rebuilds the database itself, so it never has to happen on the device. ```sh git clone --depth 1 https://github.com/Rockbox/rockbox.git cd rockbox && mkdir build-db && cd build-db ../tools/configure --target=ipodvideo --type=d && make -j$(nproc) ``` It needs a native compiler and SDL2 development headers, not the ARM cross-toolchain, and `tools/configure` detects `__aarch64__` correctly. On a distribution without `/usr/bin/perl` or `gcc-ar` — NixOS, say — patch the shebangs in `tools/*.pl` and pass `AR=ar`. Building it here rather than on the device is not merely faster. The on-device commit sorts the whole index in whatever memory `core_alloc_maximum()` can scrape together; on a fifty-thousand-track library it runs for hours or aborts outright. The scan runs against a scratch root — a real `.rockbox` beside a symlink standing in for wherever the music lands on the device — so the paths recorded are the ones Rockbox will look up, while the **bytes are read from the mirror rather than over USB**. Only the dozen `.tcd` files cross to the card. Cost, measured: the parser makes about 49 reads and 43 seeks per file, probing the head for ID3v2 and the tail for ID3v1. On a local disk that is 2,000 files in half a second. Over SMB, readahead absorbs most of the reads but the opens and the head/tail split are real round trips, so a first full scan is minutes rather than seconds. It is a one-time cost: the builder is incremental, and the scratch root is kept between runs, so a later pass over unchanged files does no metadata reads at all. If minutes is still too many, run the builder where the mirror is local — on the NAS — and copy the `.tcd` files across. There is nothing to parallelise: the tool is single-threaded, and two instances cannot produce one database. ### If the sync is interrupted No partially copied track is ever left under a name Rockbox would play. rsync writes to a hidden temporary file and only renames it into place once the file is complete, and *"by default, rsync will delete any partially transferred file if the transfer is interrupted"*. `--partial` is deliberately not used, and there is a test asserting it never will be. After an unclean kill or a power cut a hidden `.track.mp3.XXXXXX` can survive. It is not playable, it is not in the source, and the next run's `--delete` removes it. Interrupting does not, by itself, endanger the filesystem. The kernel flushes dirty pages within `dirty_expire_centisecs` — thirty seconds by default — and `umount` always syncs before it returns. Losing data needs you to interrupt, *and* pull the card inside that window, *and* skip the unmount. The script still traps `INT` and `TERM` and flushes and unmounts on the way out, exiting 130. Not because a Ctrl-C is dangerous, but because it removes the manual step and makes the exit deterministic — you get the same "safe to disconnect" either way, rather than having to remember which path you took. Both paths call the same function, so they cannot drift apart. What genuinely does lose data is pulling the cable or the card without unmounting at all, interrupted or not. FAT32 has no journal. Wait for the unmount line. ### Making it faster over a network mount The transfer is metadata-bound, not throughput-bound: 49,600 files means 49,600 round trips, and the counting pass doubles that. In rough order of what it is worth doing: | Lever | Why | | ----- | --- | | Mount the source with `actimeo=60,cache=loose` | SMB defaults to a **one second** attribute cache, so nearly every `stat` goes to the wire — twice, once per pass. This is the single biggest change and it is a mount option, not an rsync flag. | | Put the card in a reader for the first load | USB 2.0 through an iPod in disk mode is the floor for the destination. No amount of source tuning gets past it. | | Counting is off by default | The percentage costs a second full traversal of both trees. On a FAT card of fifty thousand files that is slower than the transfer. `-P` asks for it. | | `--whole-file`, `--omit-dir-times` | Already set. The first stops rsync checksumming destination files it is about to overwrite whole; the second drops a setattr per directory, 6,150 of them. | **NFS instead of SMB** is worth trying but is not the big win it looks like. Its attribute caching defaults are far more generous than SMB's — `acregmax` of sixty seconds against `actimeo=1` — which is precisely the gap that `actimeo=60` closes on the mount you already have. Bulk read throughput between the two is much of a muchness on a gigabit link. Try the mount option first; it is one line and needs no change on the NAS. And if the destination is the iPod rather than a card reader, none of this matters much: the source can feed data faster than USB 2.0 through an iPod will take it either way. The unmount is the point of doing this in a script. FAT32 has no journal and the device is reached through disk mode, so an interrupted write is corruption that needs `fsck.vfat` from another machine. `submit_scrobbles.py` sends what was played to Last.fm and sets the logs aside. It reads **Rockbox's own `playback.log`**, which core Rockbox writes whenever "play log" is enabled, with no plugin running. Each line is `timestamp:elapsed_ms:length_ms:path` — a path and nothing else, which is why the on-device scrobbler plugin exists at all: reading tags back off the player is slow. Off the mirror it is free, so `--mirror` lets the conversion happen here and the plugin never has to be run. A play counts as listened at half the track's length, the same fraction the plugin uses, so the two cannot disagree about what a play was. It still reads a `.scrobbler.log` if the plugin has been run and left one. Rockbox writes `/.scrobbler.log` in AUDIOSCROBBLER 1.1 format, one tab-separated line per track rated `L` for listened or `S` for skipped; only the listened ones are sent. It runs **before** the copy, since the plays already happened and a failed transfer is no reason to lose them. Two things are unlike every other Last.fm call in these projects. Scrobbling is a *write* method, so it needs `LASTFM_API_SECRET` and a session key obtained once through the browser, not just the read-only key. And on a target with no real-time clock Rockbox writes `/.scrobbler-timeless.log` with every timestamp set to zero; those are counted and reported but never submitted, because scrobbling them would mean inventing when they happened. The log is renamed rather than deleted once accepted. If Last.fm quietly dropped something, the evidence is still on the device. `check_fat32.py` reports paths a FAT32 device will not accept — reserved characters, trailing dots and spaces, over-long components and paths, and names colliding case-insensitively. Run it against the mirror **before** an rsync: rsync reports the failures too, but scattered through fifty thousand files where they are easy to miss. Exits non-zero when it finds anything, so it can gate a script. ## Tests ```sh docker build --target test . # what CI runs pytest # needs ffmpeg and pytest on PATH ``` The suite runs real ffmpeg encodes rather than mocking them. The interesting failures are in what ffmpeg actually does with tags, cover art and container formats, and a mock cannot fail that way — which is also why CI runs the tests _inside the image_, against the ffmpeg that ships, rather than against whatever the build runner provides. The published image is the `runtime` stage and carries neither the tests nor pytest. Run them directly instead if you prefer; they skip when ffmpeg is absent. On a Nix machine: ```sh nix shell nixpkgs#python3Packages.pytest nixpkgs#ffmpeg -c pytest ``` ## FAT32 and Rockbox `--fat32-safe` names mirror files so a FAT32 device will accept them. Off by default, because turning it on renames files and that should be a decision rather than a surprise. What it handles, per path component: | Problem | Treatment | | ------------------------------- | ------------------------------ | | `< > : " \ \| ? *` and control characters | replaced with `_` | | trailing dots and spaces | stripped — FAT eats them silently, so the name round-trips as a different name | | a component left empty | becomes `_` | | names differing only in case | detected and reported; one wins, as with any other collision | `Dada Life - Kick Out the Epic Motherf**ker` is a real example from a real library. Without this it simply never arrives on the device. **Turning it on does not re-encode anything.** Every track whose name held a reserved character changes path, and re-encoding those would be hours of work producing files that already exist byte for byte. The run moves them instead, and says so. Prune then finds nothing to remove because nothing was left behind. Renames are counted apart from encodes in the pass summary, and `--dry-run` reports `would rename` rather than `would encode` — the difference between the two is a minute against an afternoon, so a preview that conflated them would be worse than no preview. A dry run also does not list the pre-rename files as orphans: nothing was moved, so they are still there, but they are what a real run would move rather than what it would delete. ### Path length Rockbox's `MAX_PATH` is 260, from `firmware/include/fs_defines.h`, and it bounds the path *as the device sees it*. The directory the mirror is copied into comes out of the same budget, so `--device-prefix` (default `/Music`) is subtracted from `--max-path` to get what a mirror-relative path may spend: | Destination on the device | Mirror-relative budget | | ------------------------- | ---------------------- | | `/Music/` | 253 | | the card root | 259 | Worth being exact about, because a checker that measures mirror-relative paths against the flat 260 quietly passes everything from 253 to 260 — and those are the paths most likely to be near the edge in the first place. Over-budget paths are shortened from the **deepest component outward**: the track name carries the least navigational value and the artist directory the most, so the filename goes first and the artist is touched only if nothing else will do. A component is cut **from the middle**, not the end, because of how these names are built. Lidarr writes `Artist - Album - 07 - Flamethrower.mp3` inside a directory already named for that artist and album, so a long album title appears three times in one path and the informative part — the track number and title — is at the very end. Cutting from the end throws exactly that away: ``` before King Gizzard & the Lizard Wizard - PetroDragonic Apocalypse; or, Dawn of Eternal Night - An Annihilation of Planet Earth and the Beginning of Merciless Damnation - 07 - Flamethrower.mp3 after King Gizzard & the Lizard~c526~ginning of Merciless Damnation - 07 - Flamethrower.mp3 ``` Two thirds of the remaining room goes to the tail, since the head is usually a restatement of the directory it sits in. A shortened component gains four hex digits of the original name: two names sharing both a head and a tail would otherwise produce the same string, and a silent collision between two tracks is worse than an ugly filename. The result is stable: the same source always produces the same shortened name, so a pass does not rename what the previous pass wrote. A path too deeply nested to fit without reducing every component to nonsense is left alone and reported instead. ### Album art Rockbox looks for cover art **on the filesystem** — `cover.jpg`, `folder.jpg` and friends beside the track or in its parent — and that search never touches the picture embedded in the tag. So a JPEG cover found beside the source is now copied into the mirror as `cover.jpg`, in addition to being embedded. The iPod firmware reads the embedded one; Rockbox reads the file. Both are satisfied. A cover left behind in a directory whose tracks have all gone is pruned, or the directory would never look empty and never be removed. ## Getting the result onto an iPod The mirror is just a directory of MP3s, so any client will do: - **macOS.** Add the mirror's SMB share to the Music app with _Copy files to Music Media folder_ and _Keep Media folder organised_ both **off**. The Mac then stores a library database and nothing else. Keep the share mounted at a stable path — if it is missing when Music opens, every track shows `!`. - **Linux.** Rhythmbox links `libgpod` and handles iPod sync. An iPod Video (5th generation) predates the models whose database has to be signed, so no firmware-hash trickery is needed. - **Rockbox.** No database to write at all — it reads a plain directory tree and builds its own index from tags. Copy the mirror across with rsync: ```sh python3 tools/check_fat32.py /mnt/tank/media/music-mp3 # before, not during rsync -rtv --delete --modify-window=2 \ /mnt/tank/media/music-mp3/ /media/IPOD/Music/ ``` `--modify-window=2` because FAT stores modification times to two-second resolution; without it rsync re-copies the entire library on every run. `-rt` rather than `-a` because owners, groups and permissions mean nothing on FAT and asking for them only produces errors. Do not route this through Rhythmbox. Its `rb_ipod_helpers_is_ipod()` reads `access-protocols` from media-player-info first and returns true without looking at the filesystem at all, so an iPod in disk mode is identified by its USB id and managed as an iPod — writing a database Rockbox does not want. Deleting `iPod_Control` does not change this. Either use rsync, or untick Preferences → Plugins → Portable Players - iPod. Faster still for the first bulk copy: take the card out of the iFlash adapter and use a card reader. Fifty thousand files over USB 2.0 through an iPod is a long evening, and it avoids Rockbox's USB stack entirely. Neither client transcodes at sync time; they copy finished MP3s. Two device-side details worth knowing: the iPod reads cover art from the file's tags and ignores `folder.jpg`, which is why art is embedded here; and volume levelling on the device uses iTunes' Soundcheck tag, not ReplayGain, so ReplayGain tags in the source are not carried over as such.