Subtitle extraction is I/O-bound: subtitle packets are interleaved across the
whole container, so ffmpeg must read the entire file to demux a complete track
(measured ~57 MB/s reading a 60GB remux over ~75 MB/s NFS → ~14 min for the
full read). Doing that once per track meant N full reads of a huge file.
ExtractSubtitlesVTTMulti demuxes the container ONCE and routes every text track
to its own WebVTT output, so an N-text-track file costs one read instead of N.
The prewarm now enqueues one job per file (all its text indices) and raises the
per-file deadline to 45 min so even ~200GB remuxes finish the single read in the
background (idempotent; the on-demand /sub keeps its 60s fallback). Thumbnails
are unaffected — a keyframe seek reads a tiny slice (~0.7s even on 60GB).
Pre-extract the file panel's sample frames (10/30/50/70/90% of runtime, w=320)
during the library scan and write-through any on-demand /thumbnail request into
the hidden ".unarr/<name>.t<sec>w<width>.jpg" sidecar. The /thumbnail handler
serves a fresh sidecar instantly, so the characteristics panel and seekbar
previews stop re-running ffmpeg per request.
- mediainfo.sidecar: ThumbnailCachePath, ReadCachedThumbnail, WriteCachedThumbnail,
ExtractThumbnailJPEG (mirrors engine.buildThumbnailArgs).
- library.PrewarmSidecars: also enqueues the panel frame positions (kept in
lockstep with the web's THUMB_FRACTIONS / THUMB_WIDTH) per item with a duration.
- thumbnailHandler: cache-read → hit; miss → extract → write-through.
- config: library.cache_thumbnails (default true) + both cache toggles exposed in
the interactive 'unarr config' library menu.
Local only by design — frames are the user's own content, never uploaded.
On-demand WebVTT extraction re-ran ffmpeg on every /sub request and, for
50GB+ remuxes, couldn't finish a full text track within the 60s HTTP timeout
→ the web player got a 500 and no subtitles.
Extract each text subtitle ONCE — during the library scan (no HTTP deadline,
generous per-file timeout) and write-through on the first on-demand request —
into a hidden ".unarr/<name>.s<index>.vtt" sidecar next to the media file.
The /sub handler serves a fresh sidecar instantly (mtime-invalidated when the
media is replaced), so playback subtitles are instant and huge files work.
- mediainfo.sidecar: cache paths, mtime freshness, atomic write, ExtractSubtitleVTT,
IsTextSubtitleCodec (shared classifier, mirrors engine + web whitelists).
- library.PrewarmSidecars: bounded, idempotent, ctx-cancellable background pass
run after every scan (manual + daemon auto-scan).
- subtitleHandler: cache-read → hit; miss → extract → write-through.
- config: library.cache_subtitles (default true), wired via SetCacheSubtitles.
Local-only by design: nothing extracted is uploaded — the sidecar is the user's
own content, private to their disk.
