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Mumtaz Jahan
Mumtaz Jahan

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How I Fixed Jenkins Built-In Node Offline Issue on EC2

#jenkins #devops #aws #beginners

Problem: Jenkins Built-In Node Showing Offline on EC2

If you have ever set up Jenkins on an AWS EC2 instance and seen your Built-In Node showing offline with builds not running — this post is for you!

Here is exactly what happened, why it happened, and how I fixed it step by step.

Understanding the Warning

When I clicked on the Built-In Node I saw this error:

Disk space is below threshold of 1.00 GiB. Only 951.90 MiB out of 956.65 MiB left on /tmp.

Jenkins monitors your server's system resources constantly. It requires:

Resource Minimum Required
Free Disk Space ≥ 1 GB
Free Temp Space /tmp ≥ 1 GB
Free Swap Space > 0 B

In my case the checks showed:

  • Free Disk Space: 22.26 GiB — perfectly fine
  • Free Temp Space /tmp: 951.90 MiB — below Jenkins threshold
  • Free Swap Space: 0 B — no swap at all

The /tmp partition was only 956 MiB total — just under Jenkins' 1 GB requirement. So Jenkins automatically took the Built-In Node offline and refused to run any builds.

Fix Option 1 — Increase /tmp Size (Best for EC2)

This is the permanent fix. We increase the /tmp mount size by editing the filesystem configuration.

Step 1: Open the fstab file

sudo nano /etc/fstab
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Step 2: Add this line at the bottom

tmpfs /tmp tmpfs defaults,size=2G 0 0
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Save and exit (Ctrl+X → Y → Enter)

Step 3: Remount /tmp without rebooting

sudo mount -o remount /tmp
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Step 4: Verify the new size

df -h /tmp
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You should now see:

Filesystem   Size   Used   Avail   Use%   Mounted on
tmpfs        2.0G   4.8M   2.0G    1%     /tmp
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/tmp is now 2 GB — well above Jenkins' 1 GB threshold! ✅

Fix Option 2 — Quick Jenkins Restart (Temporary Fix)

If you just need Jenkins back online quickly:

sudo rm -rf /tmp/*
sudo systemctl restart jenkins
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This clears temp files and forces Jenkins to recheck the threshold. Sometimes this is enough to bring the node back online temporarily.

Bonus Fix — Add Swap Space (Important for t2.micro)

On t2.micro EC2 instances, swap is 0 B by default. Jenkins warns about this too. Here is how to create a 1 GB swap file:

# Create a 1GB swap file
sudo fallocate -l 1G /swapfile

# Set correct permissions
sudo chmod 600 /swapfile

# Set up swap space
sudo mkswap /swapfile

# Enable the swap
sudo swapon /swapfile

# Verify
free -h
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Result

After applying Fix Option 1 and adding swap space the node came back online immediately!

  • Built-In Node came back Online
  • Jenkins disk space warning disappeared
  • Builds started running immediately

Key Takeaways

t2.micro has very limited resources — always check /tmp size when setting up Jenkins on a free tier EC2.

Never ignore Jenkins resource warnings — they directly affect whether your node stays online.

Fix Option 1 is permanent, Fix Option 2 is temporary. Always go with Option 1 for a stable setup.

Always add swap on t2.micro — it prevents a lot of memory-related Jenkins issues down the line.

*Setting up Jenkins on EC2? Drop your questions in the comments *

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