# The volume manager: the plan

*2026-08-09. Executes the settled design in
[storage-architecture.md](file-system-development/storage-architecture.md) and
[storage-design-rationale.md](file-system-development/storage-design-rationale.md)
(decisions 1–8). Track discipline as always: one commit per coherent step, suite
green at phase boundaries, every new test shown to fail against the old
behavior, one QEMU suite at a time, work on main.*

**No open decisions.** Decision 4 is settled in the rationale as per-sender
range confinement at the provider on one serving endpoint — the badge-scoped
provider pattern the xHCI bus already uses, applied to blocks. The volume
manager sets each filesystem process's range; the driver clamps and translates
every transfer by the sender's kernel-stamped badge; filesystems address
volume-relative LBAs from 0 (on the confined path the FAT engine's `base_lba`
resolves to 0; the field and the engine's own MBR walk remain as now-inert
legacy, the authoritative walk living in the volume manager's `partition.zig`).
Every other decision the phases below execute is recorded in the
rationale (decisions 1–8); nothing in this plan waits on a choice.

## V0 — `fs_unmount` ownership (the defect fix)

The kernel records the mounting process on each mount slot; `fs_unmount` is
refused for any caller but the owner (the `/protocol` special case stays).
Death cleanup is unaffected (the lazy sweep is not an unmount). Discrimination:
a fixture unmounting a prefix it does not own must be refused — fails against
today's kernel, which lets any process unmount anything.

## V1 — the filesystem harness (extraction, no behavior change)

fat's 434-line shell becomes `library/file-system/harness` (name per
convention): establishment, the badge-scoped open-node table, the nine
vfs-protocol handlers, mount registration, bring-up/teardown. fat becomes
engine + on-disk + a thin main wiring the harness. Suite green is the gate;
nothing observable changes. This lands FIRST so every later phase touches the
harness once, not fat and the harness both.

## V2 — the driver mechanism: ranges, confinement, presence

- Block protocol additions (appended; numbering holds): `define_range`
  (volume-manager-only in practice — see grants), and the pushed
  `medium_changed` event (present/absent + change counter).
- usb-storage: per-badge range table (clamp + translate per sender), range
  definitions from the volume manager, and presence: a slow idle-time
  TEST UNIT READY poll plus sense-key inspection on failed transfers, emitting
  `medium_changed` on transitions. QEMU test lever: `eject` /
  `blockdev-remove-medium` against a `removable=on` usb-storage device; if
  QEMU's model refuses, the fallback drill is device_del/add of the whole
  stick (the H-matrix already proves that path) and presence gets its real
  test on the bench with a card reader.
- Discrimination: a fixture transferring outside its assigned range must be
  refused; fails against a driver without the clamp.

**Sequencing (as executed).** V2a landed the range clamp + its discrimination
fixture (block-range) — the security mechanism is testable in isolation. V2b
adds `medium_changed` and makes usb-storage a publisher that emits it on
presence transitions, but its END-TO-END test (eject → medium_changed →
unmount/remount) lands in V4 with the real consumer, the volume manager —
rather than a throwaway subscriber fixture V3 would immediately replace. Same
work, no duplicated scaffolding.

## V3 — the volume manager service

New binary `system/services/volume-manager`, spawned by init, serving the
(genuinely singular) name `volume-manager`. Duties, all moved OUT of fat:

- subscribe to the device manager; consumer-hello each storage provider for
  its block channel;
- probe: partition table walk (MBR now, GPT next — the walk LEAVES the FAT
  engine) and content identity (the decision-8 ladder: GPT GUID → fs UUID →
  FAT serial+label → MBR signature+index → anonymous);
- configuration: `filesystems.csv` (signature → filesystem binary) and
  `volumes.csv` (identity → mount prefix; the fstab). Boot volume identity
  recorded at first sight of `/system/configuration`;
- define ranges on the driver; spawn one filesystem process per volume
  (argv: volume id); answer each filesystem's startup hello with its volume
  channel (the reply-capability path, same as the device manager's);
- supervise: hello/mount deadline, crash-loop cap, reap on removal.

fat sheds `acquireVolume` and its device-manager grant; filesystem binaries
get `open volume-manager` only — a filesystem cannot acquire, only be given.
Grants move with the code in the same commits.

**Sequencing (as executed).** V3a (discovery+probe) and V3b (the flip: spawn +
confine + hand over the channel, single volume) landed the core. The V3c items —
the `volumes.csv` mount map, the fuller identity ladder (FAT serial, GPT GUID),
and multi-volume spawning — mainly serve the MULTI-volume drills (two-partitions,
clone-policy). The user's goal is the single-volume boot-stick removal lifecycle,
so V4's removal lifecycle runs next on the single volume, and the multi-volume
work + its drills become a documented follow-on (V3c/multi-volume). fat keeps its
hardcoded mount prefixes until the mount map lands.

## V4 — the removal lifecycle, end to end

Two triggers, one path: storage-channel death and `medium_changed(absent)`
both drive kill-the-filesystem-process + retire-its-mounts; return (device
re-report or `medium_changed(present)`) drives re-probe → respawn → remount
at the identity's prefix. The logger gains resume patience (retry flushes on
the fat cadence, never abandon) and the ring-wrap gap marker. QEMU cases:

- `volume-replug`: yank the boot stick mid-run, replug, assert remount at the
  same prefixes and the logger appending to the SAME boot-stamp tree with the
  gap marked. Discrimination: against pre-V4, fat wedges (`mounted` forever)
  and no remount happens.
- `volume-two-partitions`: a two-partition FAT image → two volumes, two
  filesystem processes, two mounts from one stick; yank once, both die; return
  once, both remount. Proves multi-volume and the reap breadth. (New image
  fixture beside make-fat-image.py.)
- `volume-clone-policy`: two sticks with identical FAT serials — first keeps
  the mapped name, second mounts suffixed, loudly logged.
- The **lifecycle conformance drill**, parameterized by filesystem: mount,
  serve, yank mid-write, verify honest loss (dirty flag set, gap said),
  replug, remount. FAT is implementation #1; the drill is the definition of
  "danos supports filesystem X".

## V5 — close-out

Full suite green; the architecture doc's *(planned)* markers flip to built;
`fs_unmount` ownership, ranges, presence, identity, and the volume manager
lose their future-tense; memory updated; Ryzen bench note: pull the stick,
watch the log gap get marked, plug it back anywhere.

**Not in this track** (recorded so absence is deliberate): GPT parsing beyond
the identity read (row exists in the prober's ladder; full GPT when a GPT
medium matters), AHCI/NVMe drivers (NVMe gated on the shm-ring data plane),
formatting/entropy, per-process namespaces.