plex: Media/ cache lives on the 3TB ext4 disk from day one, not the SSD

Bind mount is now active, not a commented Phase-2 option — Mike's call
2026-07-31 (337 GB is too large for the SSD). Quick Sync confirmed as
the transcode device; NAS account is read-only on the library shares.

plex/docker-compose.yml

@@ -28,8 +28,11 @@ services:
       # Claiming would create a NEW server identity and throw that away.
 
     devices:
-      # Quick Sync on the UHD 630. No session cap and better HDR tone mapping
-      # than the Pascal-era NVENC on the 1070.
+      # Quick Sync on the UHD 630 — DECIDED 2026-07-31. No session cap and
+      # better HDR tone mapping than the Pascal-era NVENC on the 1070, and the
+      # i915 driver lives in the kernel, so nothing breaks on updates. The
+      # GTX 1070 stays free for other work. Requires the BIOS iGPU
+      # Multi-Monitor toggle or /dev/dri does not exist.
       - /dev/dri:/dev/dri
 
     group_add:
@@ -39,36 +42,36 @@ services:
       - "${RENDER_GID:-993}"
 
     volumes:
-      # ---- Plex data directory -------------------------------------------
-      # Whole directory on the SSD: database, Metadata/ (25 GB of posters) and
-      # Media/ (337 GB of migrated preview cache). ~364 GB in ~440 GB usable.
-      # Fully supported layout — no split, no bind-mount trickery.
+      # ---- Plex data directory (SSD): database + Metadata, NOT Media/ -------
+      # The SQLite database is the most latency-sensitive thing on the box and
+      # belongs on flash. Metadata/ (25 GB of posters) rides along. The 337 GB
+      # Media/ preview cache does NOT — see the bind below.
       - type: bind
         source: /srv/plex/config
         target: /config
 
-      # ---- PHASE 2 (post-soak, optional) ----------------------------------
-      # Relocate the 337 GB preview cache to the 3 TB ext4 disk so future
-      # thumbnail growth (~28 MB per content-hour) stops consuming the SSD.
+      # ---- Preview/thumbnail cache on the 3 TB ext4 disk — NOT the SSD ------
+      # Mike's call 2026-07-31: 337 GB is too large for the 500 GB SSD, so
+      # Media/ lives on /srv/data (the former D:, now ext4) from day one.
       # Long-form syntax deliberately: the target path contains spaces, which
       # the short "a:b" form handles badly. Docker orders mounts by target
       # depth, so this correctly lands inside the /config mount above.
       #
-      # Plex does NOT support relocating this directory. Enable it only after
-      # running scripts/60-media-relocate.sh, which performs the move and then
-      # forces thumbnail generation on a single title to prove that writes
-      # succeed across the filesystem boundary. Rollback is moving it back.
-      #
-      # - type: bind
-      #   source: /srv/data/plex-media
-      #   target: /config/Library/Application Support/Plex Media Server/Media
+      # Plex does NOT support relocating this directory natively — this bind
+      # mount is the mechanism. After first start, force thumbnail generation
+      # on one title to prove writes succeed across the filesystem boundary
+      # (EXDEV) — that failure mode can only be tested, not reasoned about.
+      - type: bind
+        source: /srv/data/plex-media
+        target: /config/Library/Application Support/Plex Media Server/Media
 
-      # ---- NAS media, mounted at the SAME paths the database stores -------
+      # ---- NAS media, mounted at the SAME paths the database stores ---------
       # The remap rewrites \\korval\X -> /mnt/nas/X, so the container must see
       # exactly /mnt/nas/X. Six separate entries rather than one bind of
       # /mnt/nas, because a plain bind does not carry the submounts beneath it.
-      # Read-only: Plex has no reason to write to the library, and the NAS
-      # account is read-write.
+      # Read-only here AND at the mount AND at the Synology — the mediabox NAS
+      # account has no write permission on the library shares. media_pc (the
+      # box's write share) is deliberately not exposed to Plex.
       - type: bind
         source: /mnt/nas/media
         target: /mnt/nas/media