---
type: reference
description: "Dankserver's /mnt/entropy volume: the LUKS/array chain, the crypttab + fstab boot path, and the containers bound to it"
tags:
  - dankserver
timestamp: 2026-10-31
---

# Entropy Volume

`/mnt/entropy` is the ext4 filesystem on the LUKS container holding `/dev/md0`
(RAID6, seven disks). The media stack's data lives there, so the mount is a boot
dependency for those containers rather than a convenience.

## Chain

```text
/dev/md0                     <- mdadm (its mdadm.conf ARRAY is what makes md0 exist at boot)
  -> LUKS UUID=cd86c728-459e-4f3a-bcc2-41ecf84685ec   keyfile: /etc/entropy
     -> /dev/mapper/entropy  <- systemd-cryptsetup@entropy.service
        -> ext4 at /mnt/entropy  <- mnt-entropy.mount, from /etc/fstab
```

## Files that own it

| Repo file | Installs to | Produces at boot |
|---|---|---|
| `dankserver/host/crypttab` | `/etc/crypttab` | `systemd-cryptsetup@entropy.service` → `/dev/mapper/entropy` |
| `dankserver/host/fstab` | `/etc/fstab` | `mnt-entropy.mount` (mountpoint `dank:dankserver`, `0755`) |

Deploy a change by copying the file onto the host, then
`systemctl daemon-reload && update-initramfs -u`. The initramfs step is not
optional: the cryptsetup hook bakes `/etc/crypttab` into the initrd, and the
array is unlocked there, before the real root mounts. `scripts/mount-entropy.sh`
(`udisksctl unlock` + `mount`) stays as the manual path for a volume that was
never unlocked; on a correct boot it is redundant.

## Law

**`crypttab`'s device field takes the LUKS UUID, never the md array UUID.**
`blkid` prints both on adjacent lines for a RAID-backed container, and the file
shipped since 2025 carried the array UUID (`61bd6eaf-…`) where the LUKS container
is `cd86c728-…`. The generator produced a perfectly valid unit pointing at a
device that never appears, so the mapper came into existence only through a
desktop `udisks2` session — the volume looked mounted on every login while every
boot failed to mount it. The matching `fstab` line named the same wrong UUID
directly instead of the mapper.

**A LUKS keyfile is hashed whole, so a trailing newline is a different
passphrase.** The shipped `/etc/entropy` was 27 bytes — 26 characters plus `\n` —
and `cryptsetup` rejected it even after the UUID was corrected. Keyfiles stay
`0600` with no trailing newline; prove it with
`cryptsetup open --test-passphrase --key-file /etc/entropy UUID=…` *before*
relying on it for a boot.

**Prove a mount fix with the artifacts systemd generates, not with a live
mount.** The regenerated `systemd-cryptsetup@entropy.service` naming the right
device, `findmnt /mnt/entropy` sourcing `/dev/mapper/entropy`, and a matching
checksum between the repo copy and the host copy are the evidence. A mount that
exists because someone logged in proves nothing about the boot path — that
exactly describes the year this bug survived.

## Consumers

`jellyfin`, `copyparty`, and `plex` bind paths under `/mnt/entropy`; with the
volume absent they start `active (healthy)` against empty directories, which is
how a missing 91 TiB volume hides. See [[Dankserver Stack]] for the stack and
[[Storage Policy]] for what may be written here.