Pool metadata upgrade and recovery

pool.bin is cluster state. Do not delete or copy it independently on a live node. Version 3 adds a deployment identity, epoch, durable generation, and a recoverable prepare/commit record on every pool.

Compatibility matrix

Reader or writer	V1	V2	V3
Legacy V1 binary	read/write	reject	reject
V2-capable binary	read/write while mixed	read/write after the V2 fleet gate	reject
V3-capable binary	read/migrate	read/migrate	read/write; never downgrade

Leave RUSTFS_POOL_META_V3_WRITE or RUSTFS_POOL_META_V3_FLEET_CONFIRMED disabled while any running process lacks V3 support. Both must be true before an existing cluster migrates. A fresh deployment can initialize directly at V3. Once a committed V3 generation is observed, rollback to a V1/V2-only binary is not supported. Repairing a missing identity on an existing V1/V2 snapshot does not cross the V3 gate; the identity is committed as initialized while pool.bin stays on its observed legacy version.

Unknown fields are not ignored. An unsupported version or field layout is reported as incompatible and is never overwritten. A truncated or invalid payload is corrupt and may be repaired only from a verified committed replica. Conflicting identities, epochs, or transactions at the same generation are recovery required and need an operator-selected source.

Partial writes

A V3 update first conditionally writes a pending generation containing the last committed snapshot, then conditionally replaces it with the committed record. During initial bootstrap, pool.bin.identity remains initialized=false and carries a unique fresh-bootstrap nonce until that committed V3 record is verified. Restarting from an initial prepare record finishes generation 1; it never rewrites the record as V1 or V2. On restart:

• prepare-only replicas expose their previous committed snapshot;
• one committed replica makes that transaction authoritative;
• remaining pending or older replicas are repairable by the next fenced save;
• two different committed transactions at one generation stop startup.

Do not hand-edit a pending record or select a replica only because it is in pool zero. Preserve all copies when escalating recovery.

Disk replacement and metadata erasure

1. Keep a quorum of nodes online and verify the cluster is ready.
2. Stop the lagging node before replacing or erasing its metadata drive.
3. Restore storage formats and the pool.bin.identity marker from the same deployment before rejoining it.
4. Start the node and wait for it to load the verified committed generation and repair its replicas before touching another node.

An initialized identity with every pool.bin missing is recovery required. Existing storage formats with neither identity nor pool.bin are also recovery required. Format creation alone is not fresh-cluster proof. Only the elected first topology node may create a durable initialized=false bootstrap identity with a fresh-bootstrap nonce, and only after every configured disk explicitly responds that it is unformatted. An unreachable peer, a non-elected distributed node, or an existing format is not sufficient proof. All-missing pool.bin replicas are accepted only by the same startup that proved the fresh topology and persisted that pending identity. When every pool.bin is missing, a later startup must recover even if the pending identity survived. This prevents a wiped or lagging node from rebuilding empty state and overwriting the cluster. Runtime reload, rebalance activation, and rebalance worker admission all fail closed and latch the same recovery gate until the node is restarted with readable metadata.