Percona PMM mongolog versus profiler query source
Parent: Database Profiler vs Query Profiler · Published reference · snapshot 2026-10-23 · skill mongodb-cohort-2b640565deb6-pending
↓ Facts as markdownall context files
Source-anchored research for Percona PMM mongolog versus profiler query source. Run status: COMPLETED.
These notes link each claim to its source. Some citations identify archived research reports that are no longer hosted on this site. A published reference means the content is available; it does not certify independent review or accuracy.Read the editorial policy and follow the sources before relying on a claim.
Definitions
- Profiler collection is not free: MongoDB warns that profiling affects performance and storage, particularly at level 2 or with a low level-1 threshold. Mongolog avoids collection reads from system.profile, but Percona's release notes still acknowledge possible additional disk I/O. Avoid promising zero monitoring overhead. (high confidence) [source]
Structure and components
- Percona positions mongolog as a way to avoid the continuous profiler polling connection load when monitoring many databases. Its zero-connections wording must be scoped to ongoing log collection: the setup comparison explicitly lists a startup connection to discover the log path, and QAN requirements still include clusterMonitor or getCmdLineOpts privileges. This does not establish that every PMM monitoring component is connection-free. (high confidence) [source]
How it works
- PMM 3.3.0 release notes introduce mongolog to reduce connection-pool pressure when monitoring many MongoDB databases; the setup guide recommends it around 100 or more databases. (high confidence) (verified as of 2025-07-09) [source]
- Log-based monitoring does not identify one universal transport: Datadog documents getLog fallback when profiling is disabled, including a 1024-event window that can lose observations; PMM mongolog tails a file. (high confidence) (verified as of 2026-10-20) [source]
Problems, failure modes and limitations
- Mongolog requires local log-file read access and supports mongos; profiler supports remote collection, while MongoDB cannot enable its database profiler on mongos. (high confidence) [source]
- A June 2, 2026 forum user reports mongolog failing during slow WiredTiger recovery on PMM 3.7.1: getCmdLineOpts runs before MongoDB accepts connections, and the agent reaches DONE without retry. This is a user report, not a confirmed current-version defect. (low confidence) (verified as of 2026-06-02) [source]
- PMM Query Analytics can consume either MongoDB profiler records from each database's system.profile collection or slow-operation records from diagnostic log files through mongolog. QAN is the downstream analysis interface; using it does not necessarily mean enabling the database profiler. (high confidence) [source]
- PMM 3.3.0 introduced mongolog as an alternative to the default profiler source. Register a service with pmm-admin add mongodb --query-source=mongolog after configuring file-based slow-operation logging. (high confidence) (verified as of 2025-07-09) [source]
- The documented mongolog setup uses operationProfiling.mode: off with a slowOpThresholdMs value and a file logging destination. Disabling database profiling therefore does not disable slow-operation logging or prevent log-fed query analytics. (high confidence) [source]
Comparisons and alternatives
- Database profiling writes to capped system.profile collections and can add performance and disk overhead. Percona describes continuous profiler reads versus startup log-path discovery for mongolog. (high confidence) [source]
- The documented deployment tradeoff is locality: profiler collection supports remote MongoDB instances, whereas mongolog requires PMM Agent access to the diagnostic log file. The mongolog prerequisites include MongoDB write permissions and agent read permissions. (medium confidence) (verified as of 2026-07-30) [source]
Facts and statements
- PMM profiler reads per-database system.profile collections; mongolog reads diagnostic log files and can run with operationProfiling.mode off. (high confidence) [source]
- Select mongolog with pmm-admin add mongodb --query-source=mongolog; profiler remains the default query source. (high confidence) (verified as of 2026-07-30) [source]
- Do not interpret Percona's claim of identical analytics data as guaranteed identical operation coverage. A level-2 database profiler captures all operations, while the documented mongolog example logs operations beyond its threshold. MongoDB profiling filters can instead control which operations enter both outputs and override slowms and sampleRate. Coverage must be compared under the actual configurations. (high confidence) [source]
- A Percona forum reply recommends mongolog for more than 100 databases and calls it a PMM 3.2.0 feature; the PMM 3.3.0 release notes introduce mongolog, and the current setup guide says it is available from PMM 3.3.0+. (medium confidence) (verified as of 2026-10-23) [source]
- MongoDB does not provide database profiling on mongos. Percona documents mongolog support for mongos, so log-fed QAN can cover routers where the system.profile collection is unavailable. (high confidence) [source]
- Active log-file handling matters for mongolog troubleshooting. Both PMM setup and Operator documentation warn that moving or renaming active logs during rotation can interrupt collection; the PMM setup page recommends copytruncate. Treat that recommendation as documented configuration guidance, not as a verified guarantee against data loss. (medium confidence) (verified as of 2026-07-30) [source]
- Operator documentation adds pmm.querySource selection in version 1.23.0 and requires PMM 3.3.0 or newer plus logcollector.enabled for mongolog, exposing logs at /data/db/logs/. The standalone PMM setup likewise requires readable file logs. (medium confidence) (verified as of 2026-07-10) [source]
Children
- No children recorded.