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The 5GB Safety Net: How a Sacrificial File Saved Our Servers

The 5GB Safety Net: How a Sacrificial File Saved Our Servers

Running out of disk space is every sysadmin’s nightmare, especially when crucial services like MySQL stop responding and there’s no room left for diagnostics or fixes.

After one particularly stressful encounter with MySQL binary logs filling up our entire disk, our team came up with a clever preventive strategy that has since become a staple of our server management: the sacrificial file.

In this post, we’ll explain how a simple 5GB dummy file can act as your disk space safety net, preventing future headaches when things get tight.

The Problem: When Disk Space Runs Out

We were recently hit by a critical issue: MySQL’s binary logs grew unchecked, filling up the entire disk. Suddenly, our web server stopped responding, and we couldn’t even install tools like ncdu to investigate the problem because we had zero disk space left. To make matters worse, MySQL’s PURGE BINARY LOGS command hung indefinitely since the lack of space prevented it from processing.

We managed to free up some space manually by deleting unnecessary files and finally resolved the issue, but this experience made us rethink how to avoid it in the future.

The Solution: A 5GB Sacrificial File

Here’s where the sacrificial file comes in.

The idea is simple: across all of our servers, we created a large 5GB dummy file that isn’t necessary for the system to function. This file acts as a buffer, just sitting there, waiting to be deleted in case of emergency. By doing this, we ensured that if we ever ran out of disk space again, we’d have a quick and easy way to free up 5GB, giving us time to diagnose and fix the issue properly.

How to Create the Sacrificial File
Creating this safety net is incredibly simple. Here’s how we did it:

Using fallocate:

sudo fallocate -l 5G /path/to/sacrificial-file

Or, if your system doesn’t have fallocate, you can use the dd command:

Using dd:

sudo dd if=/dev/zero of=/path/to/sacrificial-file bs=1M count=5120

This creates a 5GB file on your disk that can be safely deleted whenever needed.

How the Sacrificial File Helps

Instant Disk Space When Needed: When you’re in a tight spot and disk space is at zero, you simply delete the file:

rm /path/to/sacrificial-file

Boom! You instantly free up 5GB without affecting any critical services or data. This gives you breathing room to resolve the underlying issue, such as purging logs or clearing out temp files.

Prevents System Crashes: By artificially keeping your disk a bit fuller, the sacrificial file helps you detect growing disk usage earlier. Instead of hitting 100% usage when MySQL or the web server crashes, you can monitor disk space more closely and remove the sacrificial file when it hits a certain threshold. This way, you prevent the system from halting unexpectedly.

Proactive Monitoring: The presence of this large file can be part of your monitoring strategy. If disk space starts to fill up and you need to use the file, that’s a clear sign to investigate further. It’s a great way to buy time for diagnosing what’s really going on.

Lessons Learned and Best Practices

From our experience, here’s what we recommend:

Set the Sacrificial File Size Appropriately: We went with 5GB, but the size of your sacrificial file can vary depending on your system and typical disk usage. You want enough space to give you room to troubleshoot, but not so much that it significantly eats into your usable space.

Use Disk Space Monitoring: Tools like ncdu, du, or even simple scripts can help you stay on top of disk usage trends. Combine these with the sacrificial file for a more robust approach to avoiding critical disk space issues.

Automate Log Management: For services like MySQL that produce large amounts of log data, set up automated purging policies using parameters like expire_logs_days to ensure that old logs don’t accumulate indefinitely.

Conclusion

Our 5GB sacrificial file has become a simple yet highly effective safeguard across all our servers. It provides us with a quick emergency fix when disk space runs low, preventing catastrophic crashes and giving us time to solve the underlying issues. In combination with solid monitoring and log management, this strategy has become an essential part of our server management toolkit.

Next time you’re in a disk space crunch, consider creating a sacrificial file of your own—it might just save your server from an untimely crash!

Feel free to share this simple but life-saving trick with your team, and let us know if it becomes a part of your strategy!

Author: Mark Middleton
Author:
Mark Middleton

Mark founded Zaltek in 2019 and leads our board of directors. Mark is also responsible for our technology strategy to ensure we continue to deliver the most innovative solutions to customers across the UK and beyond.

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