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Apache Fails to Start Due to Mutex/Semaphore Errors (No space left on device)

Last updated on Oct 23,2025 |

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by wynston

Introduction

When managing Apache HTTP Server (httpd) on a Linux-based system, encountering startup failures accompanied by errors such as Couldn’t create the authdigest-opaque mutex or No space left on device can indicate underlying issues with system-level semaphore limits. Apache may fail to start and log errors like the following:

AH00023: Couldn't create the authdigest-opaque mutex
AH01760: failed to create lock (opaque_lock)
(28)No space left on device

This usually indicates that the system has hit its semaphore limits or has leftover semaphores from previous Apache processes.

Prerequisites

Root or sudo access
Apache HTTP Server installed
Access to the Apache error log (e.g., /var/log/httpd/error_log)

Step-by-step Guide

Step 1: Check existing semaphores

Once log into the server, run the following command to list all current semaphore arrays and their associated processes:

ipcs -s

Step 2: Remove stale semaphores

To remove one semaphore (replace xxx):

ipcrm -s xxx

To remove all semaphores owned by Apache:

for i in `ipcs -s | grep apache | awk '{print $2}'`; do ipcrm -s $i; done

⚠️ Important Note: Only delete semaphores owned by Apache to avoid affecting other processes.

Step 3: Restart Apache

systemctl start httpd
# or
service httpd start

Check status:

systemctl status httpd

Step 4: Check and increase semaphore kernel limits

View current limits:

sysctl -a | grep kernel.sem

To increase the limits temporarily:

sysctl -w kernel.sem="500 64000 64 384"

Make it permanent:

echo "kernel.sem = 500 64000 64 384" >> /etc/sysctl.conf
sysctl -p

⚠️ Important Note: Only modify semaphore limits if you’re confident they are the root cause. Misconfiguration may affect other services.

Conclusion

This guide resolves Apache startup issues related to mutex/semaphore limits. Clearing stale semaphores and increasing kernel limits should restore functionality.

Should you have any inquiries about the guidelines, please feel free to open a ticket through your portal account or contact us at support@ipserverone.com. We’ll be happy to assist you further.