MongoDB offers several performance metrics that are crucial for troubleshooting query performance issues. Here are the key metrics used in MongoDB query performance troubleshooting:

Query Execution Metrics

• Query Targeting: This metric helps identify if queries are using indexes effectively[2].
• Execution Time: Measures the latency of read and write operations. Read operations taking more than 100ms are generally considered slow[3].
• Scanned vs. Returned Ratio: Indicates how many documents were scanned to return the result set. A high ratio suggests inefficient queries or missing indexes[4].

Resource Utilization Metrics

• CPU Utilization: High CPU usage can indicate complex queries or insufficient indexing[2].
• Memory Utilization: Monitors the usage of RAM, which is crucial for caching frequently accessed data[2].
• Storage Metrics: Tracks disk I/O and storage capacity[2].

Database Operation Metrics

• Operation Counts: Tracks the number of insert, query, update, and delete operations per second[3].
• Connections: Monitors the number of active client connections[3].

Index Performance Metrics

• Index Usage: Measures how frequently indexes are used in queries[1].
• Index Size: Tracks the size of indexes, which can impact write performance if too large[4].

Replication Metrics

• Replication Lag: Measures the delay between operations on the primary and their replication to secondaries[4].

Cache Performance

• Cache Hit Ratio: Indicates how often requested data is found in memory cache versus disk[9].
• Dirty Cache Percentage: Shows the proportion of modified data in the cache waiting to be written to disk[9].

Latency Metrics

• Read Latency: Measures the time taken for read operations[11].
• Write Latency: Tracks the duration of write operations[11].

Profiler Metrics

• Slow Query Log: Captures queries that exceed a specified execution time threshold[6].
• Query Shapes: Provides information on common query patterns and their performance[6].

By monitoring these metrics, database administrators can identify performance bottlenecks, optimize query execution, and ensure efficient resource allocation for MongoDB deployments.

Sources
[1] Track and Troubleshoot MongoDB Performance With Datadog ... https://www.datadoghq.com/blog/mongodb-database-monitoring/
[2] How to Monitor MongoDB Performance Metrics Using the ... - GeoPITS https://www.geopits.com/blog/mongodb-performance-metrics.html
[3] MongoDB Monitoring | Beginner's Guide to MongoDB performance ... https://signoz.io/blog/mongodb-monitoring/
[4] MongoDB Performance Tuning Questions https://www.mongodb.com/developer/products/mongodb/performance-tuning-tips/
[5] How to Optimize MongoDB Queries for Performance? https://www.geeksforgeeks.org/how-to-optimize-mongodb-queries-for-performance/
[6] Analyze Query Performance - MongoDB Manual v8.0 https://www.mongodb.com/docs/manual/tutorial/evaluate-operation-performance/
[7] How to Troubleshoot Slow Queries in MongoDB - Netdata https://www.netdata.cloud/academy/how-to-troubleshoot-slow-queries-in-mongodb/
[8] How do you troubleshoot MongoDB performance issues? - Dragonfly https://www.dragonflydb.io/faq/mongodb-performance-troubleshooting
[9] Review Available Metrics - MongoDB Atlas https://www.mongodb.com/docs/atlas/review-available-metrics/
[10] How To Monitor MongoDB And What Metrics To Monitor https://www.mongodb.com/resources/products/capabilities/how-to-monitor-mongodb-and-what-metrics-to-monitor
[11] Monitoring MongoDB Performance Metrics (WiredTiger) - Datadog https://www.datadoghq.com/blog/monitoring-mongodb-performance-metrics-wiredtiger/
[12] How To Monitor MongoDB's Performance - DigitalOcean https://www.digitalocean.com/community/tutorials/how-to-monitor-mongodb-s-performance
[13] Optimize Query Performance - MongoDB Manual v8.0 https://www.mongodb.com/docs/manual/tutorial/optimize-query-performance-with-indexes-and-projections/