Understanding Why ExtentsDrop Commands Run Hourly in Azure Data Explorer

Explore the reasons behind the hourly execution of `ExtentsDrop` commands in Azure Data Explorer, even with fresh data and a 30-day retention policy.
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Understanding Why ExtentsDrop Commands Run Hourly in Azure Data Explorer

If you are using Azure Data Explorer and have noticed that ExtentsDrop commands are being executed every hour, even when your data is fresh and you have a retention policy of 30 days, you are not alone. This can raise questions about efficiency and the internal workings of the system. Let’s dive deep into why this might be happening and what you need to know.

The Problem: Frequent ExtentsDrop Commands

You recently created a fresh Kusto database with a retention policy of 30 days. It appears that the command ExtentsDrop is scheduled to run every hour, executing commands like:

[[See Video to Reveal this Text or Code Snippet]]

This command translates to dropping extents that are older than 30 days (or 720 hours). While it appears harmless, it raises the question: Why is Kusto running these commands before the retention period is reached?

Dissecting the Scenario

Effective Retention Policy

By default, when you create a database with a retention policy without altering it for specific tables, all tables inherit that 30-day policy.

This means that all extents in every table are subject to an effective 30 days for soft deletion.

Command Execution Without Impact

It's important to clarify that if there are no extents older than 30 days at the time of execution, the command will not actually drop anything. Thus, while the action runs, it does not impact your data.

Understanding the Logic Behind ExtentsDrop Commands

The command to drop extents is executed on the Data Engine service, which is responsible for managing data retention based on your defined policies.

As it stands now, Kusto has implemented the logic such that these drop commands can be triggered externally, meaning the frequency of execution could change in the future.

Why This Happens and Future Considerations

Here's a broader perspective on the behavior of Kusto regarding ExtentsDrop commands:

The execution logic for determining which extents to drop is engaged regularly to prevent data retention issues.

While it might be seen as redundant to run these commands regularly when the data is still fresh, this design ensures that the system can efficiently manage and reclaim space in anticipation of data that might exceed the retention period.

Conclusion: Keeping an Eye on Your Commands

In conclusion, while the hourly execution of ExtentsDrop commands in Azure Data Explorer can seem unnecessary, it is part of the proactive management of data retention. Kusto is designed to maintain efficiency and handle data expungement thoroughly.
Understanding this mechanism will help alleviate concerns over seemingly redundant processes while ensuring your data management is on track.

Final Thoughts

If you notice recurring ExtentsDrop commands despite having a young database, you can rest assured that this is a normal part of Kusto’s operation. As you work with Azure Data Explorer, keeping an eye on such intricacies ensures better data governance and performance management in your projects.

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