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

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-<short>.

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.

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 over USB is minutes of apparent hang, and its own progress2 percentage is computed against a list it has not finished discovering. So the script counts first — files and bytes both, a second pass over the tree, which is what a percentage and an estimate that mean something cost — and renders the rest itself. 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.

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.
-Q	Skips the counting pass entirely. Costs the percentage and the estimate, saves a whole walk of the tree.
--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 the Rockbox scrobbler log to Last.fm and sets it aside. 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

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:

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:

  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.