Logs & JTL File Analysis

Understanding the Logs Tab

The Logs tab in LoadFocus provides raw output and structured summary data from your JMeter test agents. Here you can:

  • Download all test artifacts (original and modified JMX, JTL results, JMeter log files).
  • Preview the first 100,000 bytes of any file in a text panel (download the file to see all of it).
  • Correlate sample-level details with high-level Insights, Errors, and Timeline views.

Load Testing Logs Example

Test Files Panel

On the left side you will see a Test files list per location (e.g., US East). Each file entry has a download icon:

  1. Original .jmx file The exact script you uploaded before the run.
  2. Modified .jmx file The copy LoadFocus actually ran. LoadFocus adjusts the thread groups for your run settings, points CSV Data Set paths at the files you uploaded and disables the listeners in the script (Backend Listeners stay on).
  3. JTL file (JMeter Test Results) The full CSV results of your run, one row per sample, with a header row. Contains timestamps, response times, response codes, thread names, labels, and more.
  4. JMeter log file The jmeter.log from each engine, capturing info messages, warnings, and errors.

JTL Summary View

The center panel shows the first 100,000 bytes of the selected file, which is only the start of a large JTL. Each row is a single sampler result. The columns are JMeter's defaults: timeStamp,elapsed,label,responseCode,responseMessage,threadName,dataType,success,failureMessage,bytes,sentBytes,grpThreads,allThreads,URL,Latency,IdleTime,Connect. The most useful ones:

  • timeStamp Epoch milliseconds when the sample started.

  • elapsed Response time in milliseconds.

  • label Sampler name (e.g., "HTTP Request").

  • responseCode HTTP status code or sampler return code.

  • success true/false flag indicating pass/fail.

  • failureMessage Why a sample failed, when it did.

  • bytes Payload size received in bytes.

  • URL Target endpoint URL.

  • Latency Time to first byte in milliseconds.

How to Use the Logs Data

  1. Download the JTL Click the download icon next to JTL file to fetch the full result set for offline analysis.
  2. Open in External Tools Import the CSV into Excel, Grafana, or your favorite log-analysis platform to pivot, chart, and filter.
  3. Search & Filter Use your editor's find feature (e.g., search by false in the success column) to isolate failures.
  4. Correlate with Timestamps Match JTL timestamps against the Timeline or Engine Health charts to see what was happening at that moment.
  5. Review JMeter Logs If your JTL shows unusual errors, download and inspect jmeter.log for stack traces or plugin errors. See JMeter Log File Analysis.

Best Practices

  • Download Large Files The preview shows only the first 100,000 bytes, so download the JTL for anything beyond a quick look.
  • Use External Visualization Tools like InfluxDB + Grafana or Apache Superset can turn raw JTL data into interactive dashboards.
  • Timestamp AlignmenttimeStamp is epoch milliseconds, so convert it to your time zone before comparing it with the Timeline chart.
  • Archive Results Store JTL and log files alongside your test configurations for auditability and regression tracking.
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