# Atlas Diagnostics Expert > - SKIP (description-overflow seed, Glean 1000-char cap): WiredTiger storage-engine root-cause internals — cache-fill/eviction/checkpoint/MVCC mechanics behind a live perf symptom → mongodb-expert (ref Parent: [MongoDB Expert Knowledge](https://llms-explorer.com/tree/mongodb-expert-knowledge/) · 19 facets · 92 facts · page: https://llms-explorer.com/tree/atlas-diagnostics-expert/ ## Routing detail - SKIP (description-overflow seed, Glean 1000-char cap): WiredTiger storage-engine root-cause internals - cache-fill/eviction/checkpoint/MVCC mechanics behind a live perf symptom → mongodb-expert (references/mongodb-wiredtiger-internals.md) — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#routing-detail` ## When to use this skill - Atlas diagnostics and triage workflows — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-to-use-this-skill` - internal single-pane triage tooling and adjacent internal support tools — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-to-use-this-skill` - FTDC, log, metrics, alert, and explain-plan investigation choices — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-to-use-this-skill` - KB-backed Atlas troubleshooting guidance — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-to-use-this-skill` - Designing or reviewing new Atlas diagnostic tooling — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-to-use-this-skill` ## When NOT to use this skill - Data-plane query/index/schema design not live perf troubleshooting - use mongodb-expert — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-not-to-use-this-skill` - Atlas platform config/architecture (control plane, tiers, networking, security posture) - use mongodb-atlas-expert — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-not-to-use-this-skill` - Backups, DR, migration, or security architecture - use mongodb-operations-expert — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-not-to-use-this-skill` - KB article lookup - use misc-catch-all (references/mongodb-kb.md) — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#when-not-to-use-this-skill` ## Skill guidance - Prefer documented Atlas diagnostic workflow before improvising. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#skill-guidance` - Call out what directly documented vs inferred when evidence thin. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#skill-guidance` - Use misc-catch-all (references/mongodb-kb.md) alongside when need article-level troubleshooting playbooks or customer-shareable links. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#skill-guidance` ## Sub-skill routing table - Consolidates 8 diagnostics/performance sub-skills as on-demand references - Read listed references/…md file before answering deep questions. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#sub-skill-routing-table` ## Public Atlas docs - Atlas metrics: — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#public-atlas-docs` - Atlas alerts: — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#public-atlas-docs` - Performance Advisor: — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#public-atlas-docs` ## Core principle - Move from curated summary to raw evidence: — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#core-principle` - Start with a fast curated view (an internal single-pane triage tool, Atlas UI summaries, Performance Advisor, alerts, metrics) — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#core-principle` - Gather focused artifacts (logs, FTDC, explain plans, profiler samples) — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#core-principle` - Use specialized internal analyzers when first-pass evidence is insufficient — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#core-principle` - Package findings into repeatable escalation record using Atlas Diagnostic Checklist and Template — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#core-principle` ## What the internal docs establish directly - An internal single-pane-of-glass tool is the first stop for Atlas project and cluster triage. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#what-the-internal-docs-establish-directly` - Atlas UI investigation still required for disk usage, IOPS, node state, query targeting, scan-and-order, oplog window, upgrade/election context. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#what-the-internal-docs-establish-directly` - Logs and FTDC are core raw artifacts behind deeper troubleshooting. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#what-the-internal-docs-establish-directly` ## Atlas Diagnostic Checklist - Purpose: Structured manual validation before escalation. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Inputs: Project ID, node URI, cluster/node pages, logs/FTDC download links, observed symptoms and timestamps — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Outputs: Escalation-ready summary with cluster size, node status, storage, IOPS, CPU, oplog, query-targeting, restart attempts — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Disk usage and write-blocking risk — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Whether writes still accepted — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Node down / recovering / upgrade state — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Query targeting and scan-and-order behavior — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Oplog window / fall-off risk — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - CPU pressure and OOM indicators — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Thresholds: Query targeting >100 red flag; >1000 urgent. Scan-and-order stay near 0; >25 warrants investigation. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Cautions: Some node downtime during upgrades expected. Sanitize customer data before sharing log excerpts. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-ui-atlas-diagnostic-checklist` - Beyond the checklist and the Atlas UI/metrics/Performance Advisor surfaces, MongoDB support engineers also use a set of internal-only diagnostic tools (FTDC analyzers, log analyzers, Atlas Search explain-plan tooling, and an internal debugging-tool gateway) — not detailed here since they aren't publicly available. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-diagnostic-checklist` ## Atlas metrics quick reference - High cache usage → working set or write pressure — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-metrics-quick-reference` - High disk latency / queue depth → storage bottleneck — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-metrics-quick-reference` - High connections → tier limits or pooling problem — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-metrics-quick-reference` - High execution time → query/index investigation — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-metrics-quick-reference` - Atlas alerting RBAC-gated at org/project scope; severity levels: Critical, Error, Warning, Info. Alert state is diagnostic evidence, not just notification plumbing. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-metrics-quick-reference` - Performance Advisor works from slow-query evidence and suggests indexes based on query shape. Index recommendations still need read-vs-write tradeoff review before applying. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#atlas-metrics-quick-reference` ## KB-guided troubleshooting posture - Use KB for repeatable symptom-to-playbook mapping, especially when need customer-safe article or want to confirm known Atlas issue shape. Check visibility before sharing links externally. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` - Useful KB categories for Atlas diagnostics: — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` - Connection and TLS issues — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` - Oplog sizing / falling off the oplog — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` - Disk-usage interpretation — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` - Search/vector alert interpretation — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` - Network latency investigations — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#kb-guided-troubleshooting-posture` ## Standards for building new Atlas diagnostic tooling - Prefer public Atlas Admin APIs first; use private/internal only when capability not exposed publicly. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Decide consumer model up front: internal UI, CLI/programmatic tool, or agent-facing system. One API shape not fit every consumer. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Use supported auth patterns: service accounts / OAuth, Digest for legacy Admin APIs, or approved internal auth flows. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Make RBAC explicit - role annotations required, not implied. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Add intentional rate limiting for fan-out or expensive diagnostic endpoints. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Keep telemetry privacy-safe - avoid logging request/response bodies due to PII risk. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Favor versioned and better-governed public APIs when long-term tool stability matters. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Treat logs, FTDC, sample queries, and explains as potentially sensitive customer data; minimize storage and exposure. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` - Preserve TS operational pattern: summary surface first, raw artifacts second, specialized analyzers third. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#standards-for-building-new-atlas-diagnostic-tooling` ## Directly documented - Atlas Diagnostic Checklist thresholds and escalation posture — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#directly-documented` - Atlas Diagnostic Checklist thresholds and escalation posture — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#directly-documented` - Atlas metrics / alerts / Performance Advisor high-level behavior — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#directly-documented` - Internal API/auth/RBAC/privacy constraints (not detailed here) — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#directly-documented` - Atlas metrics / alerts / Performance Advisor high-level behavior — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#directly-documented` - Internal API/auth/RBAC/privacy constraints from MMS API Landscape — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#directly-documented` ## Lightly documented or partly inferred - Internal diagnostic-tool operating detail (not covered here — those tools aren't publicly available) — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#lightly-documented-or-partly-inferred` - Whether a given internal tool is currently recommended, maintained, or only historically available — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#lightly-documented-or-partly-inferred` - Whether given internal tool currently recommended, maintained, or only historically available — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#lightly-documented-or-partly-inferred` - When extending this context, read tool's current README or operator guide before making prescriptive claims. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#lightly-documented-or-partly-inferred` - — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#lightly-documented-or-partly-inferred` ## Cross-hub map — where every MongoDB topic lives - All MongoDB knowledge split across four hubs (plus misc-catch-all for KB-article lookups via references/mongodb-kb.md). If task's deep material not in this hub's Sub-skill routing table, it is reference file under sibling hub - activate that hub or Read its references/.md directly. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#cross-hub-map-where-every-mongodb-topic-lives` - High-overlap routing notes: — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#cross-hub-map-where-every-mongodb-topic-lives` - Performance symptom triage (high CPU, cache pressure, slow queries, latency spikes) starts at atlas-diagnostics-expert, but storage-engine root-cause internals (WiredTiger cache fill / dirty trigger / eviction threads / reconciliation / checkpoints) owned by mongodb-expert - cross-load mongodb-expert/references/mongodb-wiredtiger-internals.md (and mongodb-wiredtiger.md) for depth. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#cross-hub-map-where-every-mongodb-topic-lives` - Migration symptoms vs migration execution: live-cluster diagnosis → atlas-diagnostics-expert; migration/mongosync runbook → mongodb-operations-expert. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#cross-hub-map-where-every-mongodb-topic-lives` - Atlas Search/Vector query syntax & index design → mongodb-atlas-expert; slowness triage of running search → atlas-diagnostics-expert. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#cross-hub-map-where-every-mongodb-topic-lives` - Host-OS memory tuning for self-managed mongod host (transparent hugepages disable - THP/defrag=never, vm.swappiness=1, swap sizing, kernel OOM killer and oom_score_adj, NUMA placement / interleave for WiredTiger cache, vm.max_map_count) lives under the devops-infra router's devops-linux-internals sub-hub → cross-load devops-linux-internals/references/linux-memory-numa.md. This skill owns MongoDB-side cache-pressure symptom triage; that reference owns Linux memory/NUMA mechanisms and sysctls beneath it. — source: `https://llms-explorer.com/sources/mdb-context-hub/atlas-diagnostics-expert/#cross-hub-map-where-every-mongodb-topic-lives` ## Where this helps - Triaging a live Atlas cluster issue (outage, node-down, degraded performance) and needing to know which surface to reach for first — a fast curated summary, then logs/FTDC, then a specialized analyzer. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Investigating a slow-query or query-targeting problem and deciding between the Atlas Performance Advisor, the profiler, or an Atlas Search/Vector Search explain-plan tool. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Building an escalation-ready record for a support case using the Atlas Diagnostic Checklist's documented thresholds (query targeting >100 is a red flag, >1000 is urgent; scan-and-order should stay near 0). — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Deciding whether a symptom belongs in this diagnostic workflow or should route to a sibling hub — e.g. WiredTiger cache-fill/eviction root-cause internals go to mongodb-expert, not here. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* ## How to apply this - Follow the documented core principle: start with the fastest curated view (an internal triage tool, Atlas UI, Performance Advisor, alerts), then gather focused raw artifacts (logs, FTDC, explain plans), then reach for a specialized analyzer only if the first two passes are insufficient. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Use the internal single-pane-of-glass tool as the first stop for Atlas project and cluster triage, but treat its quick diagnostics as a triage accelerator, not a replacement for validating against the Atlas UI, logs, and FTDC directly. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Match the incident type to the documented workflow — cluster health/outage starts with the internal triage tool then the checklist; query/index issues start with Performance Advisor and the profiler; log-heavy incidents lean on internal log-analysis tooling. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - When building new internal Atlas diagnostic tooling, follow this pack's own standards: prefer public Atlas Admin APIs first, make RBAC explicit, add rate limiting on expensive endpoints, and avoid logging request/response bodies due to PII risk. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* ## Antipatterns - Escalating a case straight to raw FTDC/log analysis without first checking the fast curated views (internal triage tool, Atlas UI, Performance Advisor) that would have answered the question in a fraction of the time. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Sharing raw log excerpts with a customer or in an external ticket without sanitizing customer data first, which this pack explicitly flags as a caution. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Treating a specialized analyzer's LLM-assisted output as ground truth without validating the hypothesis against the underlying explain data it was derived from. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Building new internal diagnostic tooling that logs full request/response bodies for convenience, ignoring the PII-risk guidance this pack sets as a standard for new tooling. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* ## Known issues - Several internal tools in this pack's toolkit are only lightly documented or partly inferred, so treat guidance on them as directional. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - It's not always clear from the corpus whether a given internal tool is currently recommended, actively maintained, or only historically available — the pack advises reading the tool's current README before making prescriptive claims. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - Access to most of these internal tools is RBAC-gated internally, so the workflow described here assumes access this pack doesn't itself grant. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)* - This is a routing/triage hub, not the place for storage-engine root-cause internals — WiredTiger cache-fill, eviction, checkpoint, and MVCC mechanics behind a live performance symptom route to mongodb-expert instead. — [source](https://llms-explorer.com/tree/atlas-diagnostics-expert/) *(AI-suggested, synthesized from this pack's existing facts — not extracted from a source document.)*