After upgrading one of my Jenkins servers from Ubuntu 24.04 to Ubuntu 26.04, I noticed that the Built-In Node was suddenly marked offline with a temporary disk space warning.
The warning looked similar to this:
Disk space is below threshold of 1.00 GiB.
Only 816.72 MiB out of 821.43 MiB left on /tmp.At first glance, this appears to be a disk space issue. However, the server still had plenty of free storage available.
The actual cause was a change in Ubuntu’s default /tmp behavior combined with Jenkins’ default temporary space monitoring settings.
The Symptom
On the Jenkins node page, the Built-In Node may show a warning similar to:
Disk space is below threshold of 1.00 GiB.
Only 816.72 MiB out of 821.43 MiB left on /tmp.In some configurations, Jenkins may even mark the node temporarily offline.
The confusing part is that /tmp is almost empty.
For example:
df -h /tmpFilesystem Size Used Avail Use% Mounted on
tmpfs 821M 4.0M 817M 1% /tmpThe filesystem is only using about 1% of its capacity, yet Jenkins still reports a problem.
What Changed in Ubuntu 26.04?
Ubuntu 26.04 mounts /tmp as a tmpfs filesystem by default.
You can verify this with:
findmnt /tmpExample output:
TARGET SOURCE FSTYPE OPTIONS
/tmp tmpfs tmpfs rw,nosuid,nodevYou can also check the size:
df -h /tmpExample:
Filesystem Size Used Avail Use% Mounted on
tmpfs 821M 4.0M 817M 1% /tmpUnlike a traditional disk-backed filesystem, a tmpfs filesystem uses memory and swap for storage.
Depending on the amount of available memory, the total size of /tmp may be smaller than Jenkins’ default monitoring threshold.
How Jenkins Monitors Temporary Space
Jenkins includes several Node Monitors that check system health metrics such as:
- Free Disk Space
- Free Temp Space
- Free Swap Space
- Clock Difference
- Response Time
You can view these settings at:
Manage Jenkins
→ Nodes
→ Configure Node MonitorsThe default Free Temp Space monitor settings are typically:
Free Space Threshold: 1 GiB
Free Space Warning Threshold: 2 GiBThe monitor compares available space against these fixed thresholds.
Why the Warning Appears
Consider the following example:
/tmp total size: 821 MiB
/tmp available space: 816 MiBThe filesystem is almost completely unused.
However:
816 MiB < 1 GiBTherefore Jenkins reports:
Disk space is below threshold.From Jenkins’ perspective, the monitor is working exactly as configured.
The warning occurs because the total size of the filesystem is smaller than the configured threshold.
Verify That the Server Is Not Actually Running Out of Space
Before making changes, check your root filesystem:
df -h /Example:
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 100G 25G 71G 27% /If your root filesystem still has plenty of free space, then the warning is likely related to the tmpfs size rather than actual disk exhaustion.
Recommended Fix
Open:
Manage Jenkins
→ Nodes
→ Configure Node MonitorsLocate the Free Temp Space section.
You have two common options.
Option 1: Do Not Mark Nodes Offline
Enable:
Don't mark agents temporarily offlineJenkins will continue monitoring the value and display warnings, but it will no longer automatically mark the node offline.
Option 2: Lower the Threshold
Reduce the thresholds to values that make sense for your environment.
For example:
Free Space Threshold: 256 MiB
Free Space Warning Threshold: 512 MiBThis is often sufficient when /tmp is implemented as a relatively small tmpfs filesystem.
Conclusion
In this case, Jenkins was not reporting that the server disk was full. It was reporting that the available space on /tmp was below the configured Free Temp Space threshold.
After Ubuntu 26.04 changed /tmp to tmpfs by default, the total size of /tmp may be smaller than Jenkins’ default 1 GiB threshold. Adjusting the Jenkins Node Monitor settings is usually enough to resolve the warning.
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