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+# External Failure Taxonomy
+
+Date: 2026-04-11
+Status: reference
+Purpose: Catalog real production bugs from Ceph/DRBD/Mayastor/Longhorn,
+extract semantic lessons, and map to V2 exposure and V3 prevention rules.
+
+## How to read this document
+
+Each entry answers:
+1. What happened (the failure)
+2. Why it happened (the root semantic mistake)
+3. Did V2 hit this? (or could it)
+4. What V3 rule prevents it
+
+The value is not the bugfix — it is the semantic lesson.
+
+---
+
+## Category 1: Membership / Liveness Misjudgment
+
+### 1.1 Ceph: Asymmetric heartbeat flapping
+**Source**: [Ceph #14181](https://tracker.ceph.com/issues/14181)
+
+**What**: OSDs flapped up/down continuously. Monitors saw them as dead
+(cluster network heartbeat failed), but OSDs saw themselves as alive
+(public network to monitor worked). Flap loop: mark down → mark up → repeat.
+
+**Root semantic mistake**: Heartbeat and control-plane used different
+network paths. A partial network failure created contradictory liveness
+views. Timer-based death declaration with no corroboration.
+
+**V2 exposed?**: Yes. Our fast-rejoin test showed: master didn't detect
+3-second replica disconnect because heartbeat cycle was 5 seconds.
+Recovery depended on timing, not facts.
+
+**V3 rule**: P1 (timers trigger, facts decide). Liveness observation
+on one path must not contradict identity truth on another. Primary
+probes independently of master heartbeat.
+
+---
+
+### 1.2 Ceph: Stale epoch checked before sync
+**Source**: [Ceph #22673](https://tracker.ceph.com/issues/22673)
+
+**What**: After OSD destroy + recreate, OSD refused to start — checked
+DESTROYED flag against locally-cached stale OSDMap epoch instead of
+syncing to current epoch first.
+
+**Root semantic mistake**: Membership decision made against stale local
+state. The check ran before the sync.
+
+**V2 exposed?**: Similar pattern. Our stale-cleanup race: the master's
+heartbeat handler checked the block registry (stale) before the VS had
+time to report its volumes. Entry deleted prematurely.
+
+**V3 rule**: P5 (monotonic facts win). Never validate identity against
+stale local epoch. Sync to authoritative source first.
+
+---
+
+### 1.3 DRBD: Split brain from asymmetric UUID rules
+**Source**: [DRBD 9.0.20-1 release notes](https://github.com/LINBIT/drbd/releases/tag/drbd-9.0.20-1)
+
+**What**: Split brain detected despite quorum, after resize operation.
+UUID propagation differed between diskless and disk-backed nodes.
+
+**Root semantic mistake**: Generation identifier (UUID) rules were
+asymmetric across node types. Same logical operation produced different
+identity artifacts depending on node role.
+
+**V2 exposed?**: Yes. Our replicaID format mismatch: shipper used
+`serverID`, engine used `path/serverID`, orchestrator used yet another
+format. Same identity, three representations.
+
+**V3 rule**: Single canonical identity format. `MakeReplicaID(path, serverID)`
+is the only constructor. No format conversion in hot paths.
+
+---
+
+### 1.4 Longhorn: Instance manager restart blindspot
+**Source**: [Longhorn #5809](https://github.com/longhorn/longhorn/issues/5809)
+
+**What**: After instance manager pod restart, Longhorn was unaware
+replica processes were dead for 60 seconds. Replicas appeared alive
+but were actually killed.
+
+**Root semantic mistake**: The liveness monitor died with the component
+it was monitoring. No watchdog survived the restart.
+
+**V2 exposed?**: Yes. When the primary's shipper held a dead TCP
+connection after replica restart, it didn't detect the failure until
+the next barrier attempt (~30s later).
+
+**V3 rule**: Liveness detection must be independent of the transport
+being monitored. The watchdog recheck fires on a timer regardless of
+whether Ship/Barrier calls are happening.
+
+---
+
+## Category 2: Recovery Decision Error
+
+### 2.1 DRBD: Full resync when partial would suffice
+**Source**: [DRBD 9.1.19 ChangeLog](https://github.com/LINBIT/drbd/blob/drbd-9.1/ChangeLog)
+
+**What**: After brief disconnect + reconnect, DRBD performed full
+resync of all data instead of bitmap-tracked partial resync. Multiple
+root causes across versions: bitmap loss, UUID mismatch during resync,
+UUID history slot corruption.
+
+**Root semantic mistake**: The UUID comparison that decides partial vs
+full resync was fragile against transient disconnects. A UUID change
+during active resync was misinterpreted as data divergence.
+
+**V2 exposed?**: Yes. Our "no WAL retention" caused every disconnect
+(even 3 seconds) to force full 1GB rebuild instead of catchup. The
+decision boundary (R < S → rebuild) was too coarse.
+
+**V3 rule**: P4 (deterministic recovery choice). Recovery decision
+from bounded facts only: `R >= S → catchup, R < S → rebuild`.
+SmartWAL + LBA dirty map eliminates the S boundary entirely — recovery
+is always "send dirty blocks from extent."
+
+---
+
+### 2.2 Longhorn: Rebuild sent to wrong replica (address reuse)
+**Source**: [Longhorn #5709](https://github.com/longhorn/longhorn/issues/5709)
+
+**What**: After instance-manager restart, rebuild command was sent to
+wrong replica. A new volume's replica received the same IP:port as the
+old failed replica. Rebuild targeted wrong volume.
+
+**Root semantic mistake**: Recovery command routed by network address,
+not by stable replica identity. Address reuse after restart caused
+identity collision.
+
+**V2 exposed?**: Yes. Our nil-interface panic in `senderByID` was
+caused by a sender lookup returning nil `*Sender` wrapped in a non-nil
+interface. The registry keyed by `replicaID` strings, and format
+mismatch caused lookup failures.
+
+**V3 rule**: P2 (single authority per truth domain). Replica identity
+must use stable, unique IDs (epoch + path + serverID). Never derive
+identity from reusable network addresses.
+
+---
+
+### 2.3 Ceph: Backfill cancellation crashes target OSD
+**Source**: [Ceph #21613](https://tracker.ceph.com/issues/21613)
+
+**What**: Primary sent REJECT message to backfill target to cancel
+backfill. Target crashed — REJECT is a message the target sends to the
+primary, not the other way around.
+
+**Root semantic mistake**: Protocol message repurposed in wrong
+direction. The state machine on the receiving end never expected this
+message from that sender.
+
+**V2 exposed?**: Similar pattern. Our `InvalidateSessionCommand`
+dispatch path invalidated the sender's session (semantic cleanup) which
+killed the transport path needed by the rebuild coordinator. The
+command was semantically valid but operationally destructive in context.
+
+**V3 rule**: Protocol messages have fixed sender/receiver roles.
+Recovery commands flow from primary to replica. Acknowledgments flow
+from replica to primary. Never reverse the direction.
+
+---
+
+## Category 3: Completion / Durability Illusion
+
+### 3.1 Longhorn: Corrupted replica kept, healthy one discarded
+**Source**: [Longhorn #7425](https://github.com/longhorn/longhorn/issues/7425)
+
+**What**: After rebuild timeout + node failure, corrupted replica was
+kept and healthy replica was discarded. Autosalvage selected replicas
+by `healthyAt` timestamp — corrupted replica had a valid timestamp from
+before the rebuild started.
+
+**Root semantic mistake**: Health metadata was not invalidated when
+rebuild started. The metadata said "healthy" but the data was corrupt
+from a partial rebuild.
+
+**V2 exposed?**: Yes. Our double-event bug: `SessionCompleted` emitted
+on ack arrival, then `SessionFailed("sender stopped")` emitted during
+executor cleanup. The system briefly declared success then reverted to
+failure.
+
+**V3 rule**: A3 (ack arrival ≠ terminal success). Health metadata must
+be invalidated at rebuild START, not just at rebuild FAILURE. Terminal
+success is emitted only after executor/session closure completes.
+
+---
+
+### 3.2 Longhorn: Healthy replica deleted due to address collision
+**Source**: [Longhorn #9216](https://github.com/longhorn/longhorn/issues/9216)
+
+**What**: In a 3-replica degraded volume, the last healthy replica was
+deleted. A stopped replica was assigned the same port as a previously
+deleted one. The engine saw two replicas at the same address and
+deleted the "redundant" one — which was the only healthy copy.
+
+**Root semantic mistake**: Replica identity derived from network address.
+Port reuse after restart created identity collision. Two physically
+different replicas appeared as the same entity.
+
+**V2 exposed?**: Yes. The `Counter` field in SeaweedFS upstream's
+`data_node.go` is a workaround for the same problem — two heartbeat
+streams for the same IP:port after fast restart.
+
+**V3 rule**: Replica identity = `(epoch, volumePath, serverID)`.
+Immutable, never derived from ports or transport addresses.
+
+---
+
+### 3.3 Ceph: Rolling restarts orphan objects
+**Source**: [Ceph #52385](https://tracker.ceph.com/issues/52385)
+
+**What**: After restarting all nodes sequentially without waiting for
+`HEALTH_OK`, PGs entered `recovery_unfound` with permanently lost
+objects. Objects existed on stray OSDs excluded from the acting set.
+
+**Root semantic mistake**: Peering completion was not waited for before
+proceeding with the next restart. Partial membership view during
+peering excluded OSDs that had the data.
+
+**V2 exposed?**: Similar risk. If the master's primary refresh isn't
+delivered before the next heartbeat cycle, the primary operates with a
+partial roster. The rebuild may target the wrong set of replicas.
+
+**V3 rule**: Recovery operations must wait for complete membership view.
+Never start recovery with a partial roster.
+
+---
+
+## Category 4: Ordering / Race Conditions
+
+### 4.1 DRBD: Write races with very short resync
+**Source**: [DRBD 9.2.14 ChangeLog](https://github.com/LINBIT/drbd/blob/drbd-9.1/ChangeLog)
+
+**What**: A write during a very short resync was not resynced, leaving
+replicas divergent. The resync marked the region "clean" while the write
+was in-flight.
+
+**Root semantic mistake**: Region bitmap clear was not ordered after
+concurrent write drain. The bitmap said "synced" but an in-flight write
+had not been applied to the peer.
+
+**V2 exposed?**: Our rebuild bitmap has the same structure. The rule
+"WAL-applied wins over base-applied" handles this: the bitmap is SET
+by the live lane, and base lane checks before installing. But a race
+between bitmap check and base install could produce the same bug if
+the lock granularity is wrong.
+
+**V3 rule**: Recovery completion must not mark a region clean while
+writes are in-flight to that region. Drain before clear.
+
+---
+
+### 4.2 DRBD: Parallel connection handshake interference
+**Source**: [DRBD 9.1.23 ChangeLog](https://github.com/LINBIT/drbd/blob/drbd-9.1/ChangeLog)
+
+**What**: In 3+ node setups, parallel connection establishment left one
+connection in `WFBitMapT/Established` — an inconsistent half-syncing
+state.
+
+**Root semantic mistake**: Per-connection state transitions were not
+isolated. One connection's handshake interfered with another's.
+
+**V2 exposed?**: Our `ShipperGroup` manages multiple shippers. If
+`ProbeReconnectAll` probes all replicas in parallel, one probe's state
+change could interfere with another's if they share state.
+
+**V3 rule**: Per-replica state must be fully isolated. No shared
+mutable state between recovery sessions for different replicas.
+
+---
+
+### 4.3 Longhorn: Zombie engine from rapid attach/detach
+**Source**: [Longhorn #11605](https://github.com/longhorn/longhorn/issues/11605)
+
+**What**: Attach + detach within 1 second left an orphaned engine
+process. Detach happened before the instance appeared in the CR,
+so nothing knew to clean it up.
+
+**Root semantic mistake**: Deletion checked once and gave up.
+The target wasn't visible during that single check window because
+of CR sync delay.
+
+**V2 exposed?**: Our `cancelAndDrain` with 5-second timeout is the
+same pattern. If the catch-up goroutine doesn't exit within 5s, we
+abandon it. The zombie goroutine may hold resources indefinitely.
+
+**V3 rule**: Cleanup must retry until confirmed absent. A single
+check-miss is not proof of absence.
+
+---
+
+### 4.4 Mayastor: Internal/external state divergence on fault
+**Source**: [Mayastor #549](https://github.com/openebs/mayastor/issues/549)
+
+**What**: Faulting a nexus child removed it from internal structure but
+NOT from external child list. Destroy caused state machine assertion
+failure because internal and external representations disagreed.
+
+**Root semantic mistake**: Partial cleanup. Internal state changed but
+external-facing state was not updated atomically.
+
+**V2 exposed?**: Our sender registry vs shipper state divergence.
+`InvalidateSessionCommand` nulled the sender's session but the shipper's
+transport state was unchanged. The registry said "no session" but the
+shipper still had a live TCP connection.
+
+**V3 rule**: State changes must be atomic across all representations.
+If you invalidate a session, both the engine state AND the transport
+state must reflect it.
+
+---
+
+## Category 5: Background Work Corrupts Semantics
+
+### 5.1 Ceph: Recovery priority starvation
+**Source**: [Ceph #62811](https://tracker.ceph.com/issues/62811)
+
+**What**: PGs stuck in `backfilling` indefinitely. Recovery work was
+continuously preempted by peering events and sub-op reads from other
+PGs. Recovery could never make forward progress.
+
+**Root semantic mistake**: No guaranteed forward-progress for recovery.
+Each individual preemption was "correct" but infinite preemption = no
+recovery = semantic failure.
+
+**V2 exposed?**: Our recovery goroutine competes with heartbeat
+processing on the same thread. If heartbeat processing takes too long
+(e.g., many volumes), recovery stalls.
+
+**V3 rule**: Recovery must have guaranteed forward-progress slots that
+cannot be infinitely preempted. Rate-limit interruptions.
+
+---
+
+### 5.2 Mayastor: Rebuild thundering herd after node loss
+**Source**: [Mayastor #1714](https://github.com/openebs/mayastor/issues/1714)
+
+**What**: When 1 node went down in a 3-node cluster, all volumes
+attempted rebuild simultaneously. Connection failures cascaded because
+rebuild descriptors couldn't acquire IO channels. Each failed rebuild
+faulted its child, and retry created a thundering herd.
+
+**Root semantic mistake**: No admission control for concurrent rebuilds.
+Automated remediation without backoff worsened the overload.
+
+**V2 exposed?**: Currently only 1 volume in tests. With 100+ volumes,
+simultaneous rebuild after a node failure would create the same
+thundering herd on our rebuild server.
+
+**V3 rule**: Rebuild scheduling after node failure must be staggered.
+Maximum concurrent rebuilds per node. Exponential backoff on failure.
+
+---
+
+### 5.3 Mayastor: HA feedback loop under shared pool load
+**Source**: [Mayastor #1331](https://github.com/openebs/mayastor/issues/1331)
+
+**What**: Shared DiskPool under concurrent IO from 6 pods triggered
+timeouts. HA interpreted timeouts as node failure, initiated target
+replacement, generating more IO, causing more timeouts. Positive
+feedback loop.
+
+**Root semantic mistake**: HA reaction to resource exhaustion did not
+include backoff. The "fix" (replace target) consumed more resources
+than the failure it was trying to fix.
+
+**V2 exposed?**: If the rebuild server saturates the disk with base
+block reads while the primary is also serving live writes, the same
+resource exhaustion → timeout → retry loop could occur.
+
+**V3 rule**: HA/rebuild reactions must detect resource exhaustion and
+back off. Automated remediation without admission control cascades.
+
+---
+
+## Mapping to V3 Anti-Patterns
+
+| External Lesson | V3 Anti-Pattern | V3 Principle |
+|---|---|---|
+| Timer-based death without corroboration | A1 | P1: timers trigger, facts decide |
+| Stale epoch used for membership decision | A1 | P5: monotonic facts win |
+| Transport error → full rebuild | A2 | P6: transport ≠ semantic overreach |
+| Ack → success before close | A3 | P2: terminal authority = executor close |
+| Address reuse → identity collision | A4 | Stable IDs, never address-derived |
+| Health metadata survives rebuild | A5 | P3: projection is derived only |
+| Partial cleanup leaves split state | A4 | Atomic state changes across all representations |
+| Priority starvation blocks recovery | A6 | Guaranteed forward progress |
+| Thundering herd of rebuilds | A6 | Admission control + stagger |
+
+---
+
+## "Would V2 Have This Bug?" Checklist
+
+Use this for self-audit. For each external bug, ask: could our system
+hit the same failure class?
+
+| # | Failure Class | V2 Exposed? | Status |
+|---|---|---|---|
+| 1 | Heartbeat timing = semantic death | Yes (fast rejoin) | Fixed: primary probes independently |
+| 2 | Stale local state for membership check | Yes (stale-cleanup race) | Fixed: RegisteredAt grace period |
+| 3 | Asymmetric identity rules | Yes (replicaID format mismatch) | Fixed: resolveEngineReplicaID |
+| 4 | Liveness monitor dies with target | Yes (shipper holds dead conn) | Partially fixed: watchdog recheck |
+| 5 | Transport error → full rebuild | Yes (no WAL retention) | Designed: SmartWAL + LBA dirty map |
+| 6 | Address reuse → wrong target | Not yet (single volume tests) | Risk at scale: need stable IDs |
+| 7 | Ack = terminal success | Yes (double event) | Fixed: suppression + manual OnRebuildCompleted |
+| 8 | Health metadata survives rebuild | Not yet | Risk: need to track |
+| 9 | Partial cleanup (sender vs session) | Yes (sender registry) | Fixed: emitTerminal=false |
+| 10 | Recovery starvation | Not yet | Risk at scale with many volumes |
+| 11 | Rebuild thundering herd | Not yet | Risk at scale: need admission control |
+
+---
+
+*This document is updated as new external cases are discovered.*