116 lines
6.4 KiB
Markdown
116 lines
6.4 KiB
Markdown
# The volume manager: the plan
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*2026-08-09. Executes the settled design in
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[storage-architecture.md](file-system-development/storage-architecture.md) and
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[storage-design-rationale.md](file-system-development/storage-design-rationale.md)
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(decisions 1–8). Track discipline as always: one commit per coherent step, suite
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green at phase boundaries, every new test shown to fail against the old
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behavior, one QEMU suite at a time, work on main.*
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**One refinement of decision 4, flagged for sign-off rather than silently
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applied.** The decision said endpoint-per-volume. The service harness serves one
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endpoint per process, and kernel-ipc has no wait-on-many; a driver serving N
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range endpoints would need threads or a multi-endpoint harness — real machinery,
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none of it needed for the security property. The property ("a channel carries
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exactly the authority it grants") is delivered instead by **per-sender range
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confinement**: every packet already arrives with the kernel-stamped,
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unforgeable badge; the storage driver keeps a per-badge range (set by the
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volume manager, which spawned the filesystem process and knows its id) and
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clamps-and-translates every transfer by the sender's range. A filesystem
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process addresses volume-relative LBAs from 0; the provider adds the base —
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offset translation at the provider, exactly the Fuchsia session-mapping shape,
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and the FAT engine's `base_lba` code is deleted rather than moved.
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Endpoint-per-volume can still arrive later with a multi-endpoint harness; the
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wire contract does not change either way. **If this refinement is wrong, say so
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before V2.**
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## V0 — `fs_unmount` ownership (the defect fix)
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The kernel records the mounting process on each mount slot; `fs_unmount` is
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refused for any caller but the owner (the `/protocol` special case stays).
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Death cleanup is unaffected (the lazy sweep is not an unmount). Discrimination:
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a fixture unmounting a prefix it does not own must be refused — fails against
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today's kernel, which lets any process unmount anything.
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## V1 — the filesystem harness (extraction, no behavior change)
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fat's 434-line shell becomes `library/file-system/harness` (name per
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convention): establishment, the badge-scoped open-node table, the nine
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vfs-protocol handlers, mount registration, bring-up/teardown. fat becomes
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engine + on-disk + a thin main wiring the harness. Suite green is the gate;
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nothing observable changes. This lands FIRST so every later phase touches the
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harness once, not fat and the harness both.
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## V2 — the driver mechanism: ranges, confinement, presence
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- Block protocol additions (appended; numbering holds): `define_range`
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(volume-manager-only in practice — see grants), and the pushed
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`medium_changed` event (present/absent + change counter).
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- usb-storage: per-badge range table (clamp + translate per sender), range
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definitions from the volume manager, and presence: a slow idle-time
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TEST UNIT READY poll plus sense-key inspection on failed transfers, emitting
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`medium_changed` on transitions. QEMU test lever: `eject` /
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`blockdev-remove-medium` against a `removable=on` usb-storage device; if
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QEMU's model refuses, the fallback drill is device_del/add of the whole
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stick (the H-matrix already proves that path) and presence gets its real
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test on the bench with a card reader.
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- Discrimination: a fixture transferring outside its assigned range must be
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refused; fails against a driver without the clamp.
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## V3 — the volume manager service
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New binary `system/services/volume-manager`, spawned by init, serving the
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(genuinely singular) name `volume-manager`. Duties, all moved OUT of fat:
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- subscribe to the device manager; consumer-hello each storage provider for
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its block channel;
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- probe: partition table walk (MBR now, GPT next — the walk LEAVES the FAT
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engine) and content identity (the decision-8 ladder: GPT GUID → fs UUID →
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FAT serial+label → MBR signature+index → anonymous);
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- configuration: `filesystems.csv` (signature → filesystem binary) and
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`volumes.csv` (identity → mount prefix; the fstab). Boot volume identity
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recorded at first sight of `/system/configuration`;
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- define ranges on the driver; spawn one filesystem process per volume
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(argv: volume id); answer each filesystem's startup hello with its volume
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channel (the reply-capability path, same as the device manager's);
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- supervise: hello/mount deadline, crash-loop cap, reap on removal.
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fat sheds `acquireVolume` and its device-manager grant; filesystem binaries
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get `open volume-manager` only — a filesystem cannot acquire, only be given.
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Grants move with the code in the same commits.
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## V4 — the removal lifecycle, end to end
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Two triggers, one path: storage-channel death and `medium_changed(absent)`
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both drive kill-the-filesystem-process + retire-its-mounts; return (device
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re-report or `medium_changed(present)`) drives re-probe → respawn → remount
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at the identity's prefix. The logger gains resume patience (retry flushes on
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the fat cadence, never abandon) and the ring-wrap gap marker. QEMU cases:
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- `volume-replug`: yank the boot stick mid-run, replug, assert remount at the
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same prefixes and the logger appending to the SAME boot-stamp tree with the
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gap marked. Discrimination: against pre-V4, fat wedges (`mounted` forever)
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and no remount happens.
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- `volume-two-partitions`: a two-partition FAT image → two volumes, two
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filesystem processes, two mounts from one stick; yank once, both die; return
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once, both remount. Proves multi-volume and the reap breadth. (New image
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fixture beside make-fat-image.py.)
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- `volume-clone-policy`: two sticks with identical FAT serials — first keeps
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the mapped name, second mounts suffixed, loudly logged.
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- The **lifecycle conformance drill**, parameterized by filesystem: mount,
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serve, yank mid-write, verify honest loss (dirty flag set, gap said),
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replug, remount. FAT is implementation #1; the drill is the definition of
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"danos supports filesystem X".
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## V5 — close-out
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Full suite green; the architecture doc's *(planned)* markers flip to built;
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`fs_unmount` ownership, ranges, presence, identity, and the volume manager
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lose their future-tense; memory updated; Ryzen bench note: pull the stick,
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watch the log gap get marked, plug it back anywhere.
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**Not in this track** (recorded so absence is deliberate): GPT parsing beyond
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the identity read (row exists in the prober's ladder; full GPT when a GPT
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medium matters), AHCI/NVMe drivers (NVMe gated on the shm-ring data plane),
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formatting/entropy, per-process namespaces.
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