[BUG](Spark) .crc Files In _delta_log Are Not Cleaned Up.
[BUG][Spark] .crc files in _delta_log are not cleaned up
Delta Lake is a popular open-source storage layer that provides a set of tools for building data lakes. It is designed to work seamlessly with Apache Spark, making it an ideal choice for big data processing and analytics. However, like any complex system, Delta Lake is not immune to bugs and issues. In this article, we will explore a bug that has been reported in the Delta Lake community, where .crc files in the _delta_log directory are not cleaned up.
The problem is that .crc files are left in the _delta_log directory after the corresponding .json file has expired. This can lead to a buildup of unnecessary files in the _delta_log directory, which can cause performance issues and make it difficult to manage the data lake.
To understand the problem better, let's take a look at the observed results. We have a Delta table, and the top-level directory structure looks like this:
drwxr-xr-x 5 root root 4096 Sep 5 2024 _checkpoint
drwxr-xr-x 18 root root 4096 Apr 29 12:32 'date=2025-04-15'
drwxr-xr-x 26 root root 4096 Apr 17 01:17 'date=2025-04-16'
drwxr-xr-x 26 root root 4096 Apr 18 01:17 'date=2025-04-17'
drwxr-xr-x 26 root root 4096 Apr 19 01:17 'date=2025-04-18'
drwxr-xr-x 26 root root 4096 Apr 20 01:17 'date=2025-04-19'
drwxr-xr-x 26 root root 4096 Apr 21 01:17 'date=2025-04-20'
drwxr-xr-x 26 root root 4096 Apr 22 01:17 'date=2025-04-21'
drwxr-xr-x 26 root root 4096 Apr 23 01:17 'date=2025-04-22'
drwxr-xr-x 26 root root 4096 Apr 24 01:17 'date=2025-04-23'
drwxr-xr-x 26 root root 4096 Apr 25 01:17 'date=2025-04-24'
drwxr-xr-x 26 root root 4096 Apr 26 01:17 'date=2025-04-25'
drwxr-xr-x 26 root root 4096 Apr 27 01:17 'date=2025-04-26'
drwxr-xr-x 26 root root 4096 Apr 28 01:17 'date=2025-04-27'
drwxr-xr-x 26 root root 4096 Apr 29 01:17 'date=2025-04-28'
drwxr-xr-x 13 root root 4096 Apr 29 12:18 'date=2025-04-29'
drwxr-xr-x 2 root root 4055040 Apr 29 12:48 _delta_log
`
As you can see, there are several directories with dates in the format `date=YYYY-MM-DD`. These directories contain the data for each day, and the `_delta_log` directory contains the log files for each day.
**Expected Results**
==================
The expected result is that the `.crc` files in the `_delta_log` directory should be cleaned up after the corresponding `.json` file has expired. This would prevent the buildup of unnecessary files in the `_delta_log` directory and make it easier to manage the data lake.
**Environment Information**
=====================
Here is the environment information for the Delta Lake version, Spark version, and Scala version:
* Delta Lake version: 3.3.1
* Spark version: 3.5.5
* Scala version: 3.12
**Willingness to Contribute**
=====================
The Delta Lake Community encourages bug fix contributions. If you or another member of your organization is willing to contribute a fix for this bug to the Delta Lake code base, please indicate your willingness to contribute below:
* [ ] Yes. I can contribute a fix for this bug independently.
* [ ] Yes. I would be willing to contribute a fix for this bug with guidance from the Delta Lake community.
* [x] No. I cannot contribute a bug fix at this time.
**Conclusion**
==========
In conclusion, the bug where `.crc` files in the `_delta_log` directory are not cleaned up is a significant issue that can cause performance issues and make it difficult to manage the data lake. We have explored the observed results, expected results, and environment information for this bug. If you or another member of your organization is willing to contribute a fix for this bug to the Delta Lake code base, please indicate your willingness to contribute below.<br/>
**[BUG][Spark] .crc files in _delta_log are not cleaned up: Q&A**
**Introduction**
===============
In our previous article, we explored a bug in the Delta Lake community where `.crc` files in the `_delta_log` directory are not cleaned up. This bug can cause performance issues and make it difficult to manage the data lake. In this article, we will answer some frequently asked questions (FAQs) about this bug.
**Q: What is the cause of this bug?**
=====================================
A: The cause of this bug is not yet fully understood, but it is believed to be related to the way Delta Lake handles log files in the `_delta_log` directory. Specifically, it is thought that the `.crc` files are not being properly cleaned up after the corresponding `.json` file has expired.
**Q: How can I reproduce this bug?**
=====================================
A: To reproduce this bug, you will need to create a Delta table and write data to it using Apache Spark. You can then check the `_delta_log` directory to see if the `.crc` files are being cleaned up properly. If they are not, you can try to reproduce the issue by writing more data to the table and checking the `_delta_log` directory again.
**Q: What are the symptoms of this bug?**
=====================================
A: The symptoms of this bug include:
* `.crc` files remaining in the `_delta_log` directory after the corresponding `.json` file has expired
* Performance issues due to the buildup of unnecessary files in the `_delta_log` directory
* Difficulty managing the data lake due to the presence of unnecessary files
**Q: How can I fix this bug?**
==========================
A: Unfortunately, there is no known fix for this bug at this time. However, the Delta Lake community is actively working on resolving the issue, and a fix may be available in a future release.
**Q: Can I contribute to the fix for this bug?**
=============================================
A: Yes, the Delta Lake Community encourages bug fix contributions. If you or another member of your organization is willing to contribute a fix for this bug to the Delta Lake code base, please indicate your willingness to contribute below:
* [ ] Yes. I can contribute a fix for this bug independently.
* [ ] Yes. I would be willing to contribute a fix for this bug with guidance from the Delta Lake community.
* [x] No. I cannot contribute a bug fix at this time.
**Q: What is the impact of this bug on my data lake?**
=====================================================
A: The impact of this bug on your data lake will depend on the size of your data lake and the frequency with which you write data to it. If you have a large data lake and write data to it frequently, you may experience performance issues due to the buildup of unnecessary files in the `_delta_log` directory.
**Q: Can I prevent this bug from occurring in the future?**
=====================================================
A: Yes, you can prevent this bug from occurring in the future by regularly cleaning up the `_delta_log` directory and removing any unnecessary files. You can also try to reproduce the issue and report it to the Delta Lake community so that a fix can be developed.
**Conclusion**
==========
In conclusion, the bug where `.crc` files in the `__log` directory are not cleaned up is a significant issue that can cause performance issues and make it difficult to manage the data lake. We have answered some frequently asked questions about this bug and provided information on how to reproduce and fix the issue. If you or another member of your organization is willing to contribute a fix for this bug to the Delta Lake code base, please indicate your willingness to contribute below.