danos/docs/file-system-development/storage-architecture.md

# The storage architecture: layers, boundaries, responsibilities

> **Status:** the layered model below is the settled design
> ([storage-stack-discussion.md](../storage-stack-discussion.md) records how it
> was reached and what the surveyed systems taught). The data path — vfs
> protocol, kernel mount routing, the FAT service, the block protocol,
> usb-storage — is **built**. The volume manager, the driver's range
> mechanism, per-volume filesystem spawning, and the removal lifecycle are
> **planned**; until they land, the FAT service performs volume-manager duties
> itself (marked below). This document is the reference for both states.

## The model

One pattern, applied twice: an application is a client of a service over a
protocol; a service is a client of a driver over a protocol.

```
application
    │  vfs protocol            (routed by the kernel mount table)
    ▼
filesystem service  ──  one process per volume
    │                   inside: [file-protocol shell] ── Zig API ── [fs engine]
    │  block protocol          (channel received at spawn)
    ▼
storage driver      ──  one process per device (usb-storage per stick)
    │  usb-transfer protocol   (channel received via hello, by lineage)
    ▼
bus driver          ──  one process per controller (usb-xhci-bus)
    │  hardware
```

Three kinds of boundary, deliberately different:

- **Protocol boundaries** sit where processes meet (vfs, block, usb-transfer).
  Each side is independently restartable; channels are established by
  capability handoff, never by registry names (communication.md
  "Establishment: two planes"). Steady-state file I/O crosses exactly two —
  vfs and block.
- **Library boundaries** sit where concerns meet inside one process. The
  filesystem service's engine (on-disk format logic, host-testable, behind
  the four-function `BlockDevice` vtable) and shell (IPC serving, open-node
  table, mount registration) talk through a Zig API. No protocol between
  them: they share fate regardless — a corrupt engine corrupts the answers
  either way — so a channel there would add a hop per file operation and buy
  nothing. The seam exists for compile-time testability and reuse; the
  *process* is the restart unit.
- **Control-plane relationships** sit beside the data path, never on it. Two
  supervisors, one per layer: the **device manager** wires and revives the
  device layers (bus and storage drivers — devices only); the **volume
  manager** *(planned)* wires and revives the volume layer (filesystem
  services). Neither touches steady-state I/O.

## Who does what

**Bus driver** (usb-xhci-bus, per controller): hardware only. Enumerates,
reports children to the device manager, serves the transfer contract to its
own children's class drivers, routed by lineage.

**Storage driver** (usb-storage, per device): hardware → blocks. Speaks SCSI
Bulk-Only Transport upward from its device; serves the block protocol (five
verbs: geometry, read, write, flush, attach). **Content-blind, permanently**:
it reads block 0 only as a bring-up self-check and parses nothing — no MBR, no
GPT, no filesystem magic, ever. *(Planned)* it gains one content-blind
mechanism: **named sub-ranges** — "serve blocks [a, b) as a channel of the
same block contract". It clamps and offsets; it never knows the numbers came
from a partition table. The clamp lives here and nowhere else because a
channel must carry exactly the authority it grants: handing a filesystem the
whole disk plus a polite base offset would let a buggy or compromised
filesystem scribble the neighboring partition — the same authority-overshoot
the device-authority track eliminated for MMIO and DMA.

**Volume manager** *(planned; today the FAT service squats on these duties)*:
the policy home of the volume layer, one service, supervised by init. It
subscribes to the device manager's child events; when a storage provider
appears it consumer-hellos for the block channel, reads the partition table
and the first blocks itself (**it** is the prober), consults its
configuration, defines sub-ranges on the driver, spawns the matching
filesystem service per volume with that volume's channel, supervises it, and
decides mount placement. Its tables are CSV configuration, read by it (the
policy), enforced by nobody else:

- `filesystems.csv` — content signature → filesystem binary. Adding a
  filesystem adds a row.
- the mount map — volume identity → mount prefix. The boot volume is chosen
  by **content** (the volume carrying `/system/configuration` and
  `/system/logs`), never by port or arrival order.

**Filesystem service** (the FAT service today; one process per volume): the
proven unit — block-client + engine + file-protocol provider in one binary. It
receives its block channel at spawn; it never discovers devices. It registers
its own mounts with the kernel; its write cache lives inside the process, so a
write error is observed by the code that owns the volume and surfaces on the
owning channel (the anti-fsyncgate rule — never a system-wide dirty pool).
*(Today, interim:)* fat also acquires its own volume (first mass-storage child
by enumeration order) and hardcodes its mount prefixes; both migrate to the
volume manager.

**Kernel** (mechanism only): the mount table routes paths to backend
endpoints — resolve and redirect, never data. Remount-replace is the restart
story; a dead backend's slot is swept lazily on the next resolution.
*(Planned fix:)* `fs_unmount` gains ownership — only the mounting endpoint's
holder may unmount; today it is ungated, which is safe with one mount owner
and wrong with several.

## Adding a filesystem

1. **Write the engine**: pure Zig, no IPC, behind the `BlockDevice` vtable
   (four functions). On-disk layout in `align(1)` extern structs. Host-test it
   with `zig build test` fixtures — the FAT engine
   ([engine.zig](../../system/services/fat/engine.zig),
   [on-disk.zig](../../system/services/fat/on-disk.zig)) is the template, and
   its ~500 lines of host tests the standard.
2. **Reuse the shell**: the filesystem harness — establishment, the
   badge-scoped open-node table, the nine vfs-protocol handlers, mount
   registration, the removal path — is shared code, not per-filesystem code.
   *(Planned: extracted from fat's 434-line shell into a library before the
   second engine is written.)* An engine plus a `main` wiring it into the
   harness is a complete filesystem service.
3. **Add the configuration row**: one line in `filesystems.csv` mapping the
   on-disk signature to the binary. No other component changes: the volume
   manager probes, matches, spawns; the vfs protocol is already
   backend-neutral (a second provider serves it in production today — init's
   synthetic registry backend).
4. **Prove removal**: extend the removable-media suite (below) for the new
   filesystem — surprise-yank during writes must lose only what was
   unflushed, said honestly, with the volume consistent enough to remount.

The engine never sees: partitions (it receives a volume-shaped block channel;
base offsets are the driver's clamp), device discovery (the channel arrives at
spawn), mount policy (prefixes are handed to it), or other volumes (one
process, one volume).

## Removable media: responsibilities on removal, per layer

The design rule, learned from what Linux cannot do: there is exactly **one**
surprise-removal path — kill the filesystem process, retire its mounts,
respawn on return. No half-alive states, no `remount-ro`, no mounts that
error forever (Plan 9's dead-server wart).

| Layer | Observes | Must do | Guarantees |
|---|---|---|---|
| Bus driver | port/hub status change | tear down the device's slots (children first, recursively — built, hot-plug matrix), report `child_removed` per interface | the device tree is honest within one reconcile tick |
| Device manager | `child_removed` / reporter death | prune the child; **reap the bound driver** (built) — the storage driver for that stick dies now, not never | no zombie storage processes; re-report rebinds |
| Storage driver | its own death (it IS the removed device's driver) | nothing — dying is its removal handling; DMA/IOMMU/claims release mechanically at death | in-flight transfers fail visibly to callers, never hang |
| Volume manager *(planned)* | the storage provider's channel death / the manager's child events | kill each filesystem service of that device's volumes; retire their kernel mounts; remember the volume identity | one removal path; mounts never dangle; log persistence stops *cleanly* |
| Filesystem service | its block channel dies (`EPEER`) mid-operation, or it is killed by the volume manager | if it observes the death first: flush nothing (the medium is gone), answer in-flight requests with errors, exit; dirty write-back data is **lost and said to be lost** | the FAT dirty flag on disk marks the unclean removal; the process never serves from behind a dead channel |
| Kernel | backend endpoint death | lazy mount-slot sweep on next resolve (built); *(planned)* ownership-gated `fs_unmount` | resolution under a dead mount is `not_found`, not a hang |
| Application | `not_found` / error on paths under the vanished mount | its own error handling — the contract is honest absence, identical to the path never existing | no operation blocks forever on removed media |

**On return** the same table runs upward in reverse: the bus re-enumerates and
re-reports; the device manager respawns the storage driver (built); the volume
manager re-probes — same content, same volume identity — respawns the
filesystem service, and remounts at the same prefix; applications see the
subtree reappear. A different stick in the same port is a *different volume*
(identity is content, not port) and mounts wherever its identity says —
possibly nowhere but `/volumes/…`.

What is lost on a surprise yank is exactly the write-back window of the
filesystem service, no more: the engine owns its cache, so the blast radius of
a yank is one volume's unflushed writes, marked by the on-disk dirty flag. A
filesystem format with better crash honesty (journaling, copy-on-write — the
lesson of QNX's Power-Safe) narrows that window further and slots in as
implementation N+1 through the table above, changing nothing else.