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Dominik Schön bbbe4f8985 feat: patterns/ directory + skill-impact tracker (WikiSkill-inspired)

- New patterns/ directory with 7 initial failure-mode patterns (PAT-001..007)
- skill-impact.md audit trail for skill modifications
- _template.md for future pattern creation
- index.md updated with Patterns section
- log.md entry for this change
- Inspired by arXiv:2608.27454 (WikiSkill)

2026-08-30 11:16:19 +00:00

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pattern_id, title, category, severity, status, first_observed, last_updated, related_systems, related_solution_docs, related_skills

pattern_id

title

OPTIMIZE TABLE on Galera triggers TOI DDL → Deadlock

Symptom

Running OPTIMIZE TABLE on a Galera cluster node causes a cluster-wide stall or deadlock. The table lock propagates via TOI (Total Order Isolation) to all nodes, blocking all writes to ALL tables during the operation.

Root Cause

Galera executes DDL statements (including OPTIMIZE TABLE, ALTER TABLE, etc.) in TOI mode. This means the DDL is replicated as a global operation that blocks the entire cluster — not just the target table. For large tables, the rebuild phase can take minutes, causing apparent outages.

Mitigation

Use RSU before DDL: Switch the node to Rolling Schema Update (RSU) mode before running OPTIMIZE TABLE. This prevents cluster-wide blocking:
```
SET GLOBAL wsrep_OSU_method = 'RSU';
-- run OPTIMIZE TABLE on this node only
SET GLOBAL wsrep_OSU_method = 'TOI';  -- restore
```
Schedule during maintenance window — even with RSU, the node itself is degraded
Consider pt-online-schema-change for large tables — avoids blocking entirely

Prevention

Never run OPTIMIZE TABLE or ALTER TABLE in production without RSU on Galera
Add this check to DBA runbooks and monitoring alerts
The Galera skill (mariadb-galera-cluster-administration) documents this pitfall

Evidence

Observed during Galera cluster administration sessions (Jul 2026)
MEMORY.md entry: "OPTIMIZE TABLE=TOI DDL→Deadlock! RSU davor."
Galera cluster: nodes 300/301/302, VIP .70:3306

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