Monitor MongoDB with mongotop

Overview

This section demonstrates how to monitor MongoDB performance using mongotop, which reports how much time the server spends reading and writing per collection in real time. It includes benchmark results collected on Azure Arm64 virtual machines, providing a reference for expected latencies.

Prerequisites

• mongod is running locally and bound to 127.0.0.1 (as started earlier)
• Your long_system_load.js script is actively generating traffic in another terminal
• MongoDB Database Tools (which include mongotop) are installed

Run mongotop - terminal 2

    

        
        
mongotop 2

    

This refreshes every 2 seconds and displays per‑collection time spent on reads and writes. Press Ctrl+C to stop.

Example output

The tail end of the output should look like:

    

        
        `                           ns    total    read    write    2025-09-04T04:58:23Z
test.admin_system_version_test      5ms     2ms      3ms
    test.system_sessions_bench      5ms     2ms      3ms
                admin.atlascli      3ms     1ms      1ms
  config.system_sessions_bench      3ms     1ms      1ms
                 test.atlascli      3ms     1ms      1ms
        benchmarkDB.cursorTest      2ms     1ms      1ms
    benchmarkDB.testCollection      2ms     1ms      1ms
     config.transactions_bench      2ms     1ms      1ms
    local.system_replset_bench      2ms     1ms      1ms
          admin.system.version      0ms     0ms      0ms

                            ns    total    read    write    2025-09-04T04:58:25Z
                admin.atlascli      5ms     2ms      3ms
  config.system_sessions_bench      4ms     1ms      3ms
    test.system_sessions_bench      3ms     1ms      1ms
        benchmarkDB.cursorTest      2ms     1ms      1ms
    benchmarkDB.testCollection      2ms     1ms      1ms
     config.transactions_bench      2ms     1ms      1ms
    local.system_replset_bench      2ms     1ms      1ms
test.admin_system_version_test      2ms     1ms      1ms
                 test.atlascli      2ms     1ms      1ms
          admin.system.version      0ms     0ms      0ms

                            ns    total    read    write    2025-09-04T04:58:27Z
test.admin_system_version_test      6ms     2ms      3ms
        benchmarkDB.cursorTest      5ms     2ms      3ms
    benchmarkDB.testCollection      5ms     2ms      3ms
     config.transactions_bench      5ms     2ms      3ms
    local.system_replset_bench      5ms     2ms      3ms
                 test.atlascli      5ms     2ms      3ms
    test.system_sessions_bench      5ms     2ms      3ms
                admin.atlascli      3ms     1ms      1ms
  config.system_sessions_bench      3ms     2ms      1ms
          admin.system.version      0ms     0ms      0ms

        
    

Interpret the metrics

Before you dive into individual columns, read each line as a snapshot of where time was spent in the last interval. Start with total to spot hot namespaces, then compare read vs write to understand the workload shape. Look across multiple refreshes for trends—steady growth or a single collection dominating is a signal to investigate indexes, query shapes, or storage latency.

• ns (namespace) the database.collection being measured
• total total time the server spent servicing operations for that namespace in the interval
• read time spent on read operations such as queries and fetches
• write time spent on write operations such as inserts, updates, and deletes
• timestamp the time when the snapshot was captured

Namespaces

• benchmarkDB.testCollection core benchmark collection with balanced read/write load
• admin.atlascli tracks admin-level client activity
• benchmarkDB.cursorTest measures cursor operations during benchmarking
• config.system_sessions_bench benchmarks session handling in config DB
• config.transactions_bench evaluates transaction performance in config DB
• local.system_replset_bench tests replication set metadata access
• test.admin_system_version_test monitors versioning metadata in test DB
• test.atlascli simulates client-side workload in test DB
• test.system_sessions_bench benchmarks session handling in test DB
• admin.system.version static metadata collection with minimal activity

Benchmark summary on Arm64

For easier visualization, here is a summary of benchmark results collected on an Arm64 D4ps_v6 Azure Ubuntu Pro 24.04 LTS virtual machine.

With the MongoDB performance summary of the results on your Arm-based Azure Cobalt 100 VM, you will notice:

• Stable, low-latency behavior across all tested namespaces
• Read operations are near-instant (sub-2 ms), showing efficient query performance
• Write operations remain consistently low, supporting reliable data modifications
• System and transaction overheads are predictable, indicating a well-tuned environment for concurrent or replicated workloads

In conclusion, MongoDB operations on Arm64 are lightweight with predictable, low-latency reads and writes, confirming efficient performance on Azure Ubuntu Pro 24.04 LTS Arm64 virtual machines.