Retention Policies in Prometheus

What are Retention Policies?

Retention policies determine how long time-series data is stored in Prometheus. These policies are essential for managing storage efficiently and ensuring that you only keep the data you need for your monitoring and analysis.

Why Use Retention Policies?

Using retention policies helps to:

• Control disk usage by limiting the amount of stored data.
• Improve performance by reducing the volume of data queries.
• Comply with legal or organizational data retention requirements.

Default Retention Policy

By default, Prometheus retains data for 15 days. This can be modified by specifying a different duration in the Prometheus configuration file.

Configuring Retention Policies

To configure retention policies in Prometheus, you need to modify the command-line flags when starting the Prometheus server. The primary flag is --storage.tsdb.retention.time.

Here’s an example of setting the retention period to 30 days:

Retention Policy Examples

Example 1: Retain for 7 Days

To keep the data for only a week, use:

Example 2: Retain for 1 Year

If you want to retain data for a year, you can specify:

Example 3: Retain for 2 Hours

For temporary metrics, you might want a shorter retention:

Considerations When Setting Retention Policies

When configuring retention policies, consider the following:

• Storage Capacity: Ensure that your storage can handle the amount of data you wish to retain.
• Data Importance: Retain data based on its importance and relevance to your monitoring needs.
• Compliance Requirements: Ensure that your retention settings comply with any regulatory data retention policies.

Monitoring Retention Policies

You can monitor the effectiveness of your retention policies by checking the prometheus_tsdb_head_series metric, which provides insights into the number of active time series.

Conclusion

Retention policies are a critical aspect of managing time-series data in Prometheus. By configuring them appropriately, you can optimize storage usage, improve performance, and ensure compliance with data retention requirements.