# Percona PMM mongolog query source > Source-anchored research for Percona PMM mongolog query source. Run status: COMPLETED. Parent: [Database Profiler vs Query Profiler](https://llms-explorer.com/tree/database-profiler-vs-query-profiler/) · 4 facets · 18 facts · page: https://llms-explorer.com/tree/percona-pmm-mongolog-query-source/ ## How it works - Both PMM setup and Operator documentation warn that moving or renaming active log files can interrupt mongolog collection. Validate rotation behavior when diagnosing missing QAN data. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) - Interpret Percona’s identical-analytics wording as feature/data comparability, not guaranteed identical operation coverage. This inference follows because level-2 profiling captures all operations, whereas normal diagnostic logging follows a threshold and sample rate; filters can further change selection. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) ## Problems, failure modes and limitations - Mongolog can run with profiling mode off. Do not infer identical operation coverage: level-2 profiling records all operations, whereas slow-log collection follows configured logging selection. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) - PMM documents mongolog support for mongos; MongoDB cannot enable its database profiler on mongos, although diagnostic slow-operation logging remains available. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) - Mongolog requires file-based slow-operation logging, MongoDB write access to its log file, PMM Agent read access, and appropriate database privileges. (low confidence) — [source](https://github.com/percona/pmm/blob/main/documentation/docs/use/qan/QAN-stored-metrics.md) - A June 2, 2026 user report on PMM Server/Client 3.7.1 describes getCmdLineOpts failing during slow WiredTiger recovery, followed by STARTING → STOPPING → DONE without retry. This is an unconfirmed version-specific report, not proof of a current defect. (low confidence) (verified as of 2026-06-02) — [source](https://forums.percona.com/t/pmm3-mongodb-2-questions-about-mongodb-mongolog-agent-and-query-analytics-panel/40045) - Datadog independently documents collecting MongoDB slow operations from system.profile when profiling is enabled, or via getLog when disabled. Its warning about a 1,024-event getLog buffer belongs to that API-based collector and must not be assigned to PMM’s file-tailing mongolog source. (medium confidence) (verified as of 2026-10-30) — [source](https://docs.datadoghq.com/database_monitoring/setup_mongodb/selfhosted/) - Log collection can run with operationProfiling.mode: off and a configured slowOpThresholdMs. MongoDB must write to a file and PMM Agent must be able to read it. Disabling system.profile collection does not disable diagnostic slow-operation logging. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) ## Comparisons and alternatives - Percona attributes reduced connection contention to reading files instead of querying databases for QAN. This scopes the benefit to query collection; it does not establish that all PMM components need zero connections or that logging has zero server cost. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/release-notes/3.3.0.html) ## Facts and statements - PMM mongolog reads MongoDB diagnostic log files for QAN; the profiler source instead reads per-database system.profile collections. An analytics interface therefore need not depend on enabling the database profiler. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) - Select log collection with pmm-admin add mongodb --query-source=mongolog. The command reference lists profiler as the default, with mongolog and none as alternatives. (high confidence) (verified as of 2026-10-30) — [source](https://github.com/percona/pmm/blob/main/documentation/docs/use/commands/pmm-admin/add.md) - Official release notes introduce mongolog in PMM 3.3.0, released July 9, 2025; a forum reply naming 3.2.0 conflicts with that primary release record. (medium confidence) (verified as of 2025-07-09) — [source](https://docs.percona.com/percona-monitoring-and-management/3/release-notes/3.3.0.html) - Mongolog avoids continuous profiler polling, but still connects at startup to discover the log path; its documented privileges include clusterMonitor or getCmdLineOpts. (high confidence) — [source](https://docs.percona.com/percona-monitoring-and-management/3/install-pmm/install-pmm-client/connect-database/mongodb.html) - Operator issue 2363 reports open /data/db/mongod.log: no such file or directory with PMM3 on Operator 1.21.2, and working mongolog collection after 1.22.0 mounted the data PVC. Treat this as reporter-tested evidence, not a universal upgrade guarantee. (low confidence) (verified as of 2026-05-28) — [source](https://github.com/percona/percona-server-mongodb-operator/issues/2363) - Percona's July 23, 2026 Operator 1.23.0 announcement lists choosing mongolog as a QAN source under K8SPSMDB-1546. (low confidence) (verified as of 2026-07-23) — [source](https://www.percona.com/blog/percona-operator-for-mongodb-1-23-0-clustersync-vector-search-pvc-snapshot-backups/) - PMM distinguishes stored QAN for completed queries from real-time analytics for running operations. Selecting mongolog changes the stored-query input source; it does not itself provide an in-flight operation view. (high confidence) — [source](https://www.percona.com/blog/what-is-running-on-my-mongodb-right-now-real-time-analytics-in-pmm/) - PMM 3.3.0, released July 9, 2025, introduced mongolog as an alternative query source intended to reduce monitoring connection pressure in deployments with many databases. (high confidence) (verified as of 2025-07-09) — [source](https://docs.percona.com/percona-monitoring-and-management/3/release-notes/3.3.0.html) - Operator 1.23.1 documentation says mongolog support was added in Operator 1.23.0, requires PMM 3.3.0 or newer, and needs logcollector enabled so PMM can read logs under /data/db/logs/; select it with spec.pmm.querySource: mongolog. (low confidence) (verified as of 2026-07-10) — [source](https://docs.percona.com/percona-operator-for-mongodb/1.23.1/monitoring.html)