No space left on device: AH00023: Couldn’t create the rewrite-map mutex

From time to time, Apache will fail to start after an update or reboot of the server.

Checking the error log at /var/log/httpd/error_log will show an error like:

No space left on device: AH00023: Couldn’t create the rewrite-map mutex

Although, the error message may vary slightly and might show something like:
(28)No space left on device: Cannot create SSLMutex
or
(28)No space left on device: Couldn't create accept lock


This is almost always caused by the semaphore limit of the OS, and Apache not properly cleaning up after itself. Luckily this is a simple problem to fix. Simply execute the following command via SSH or console:

ipcs -s | awk -v user=apache '$3==user {system("ipcrm -s "$2)}'
  • apache, error, mutex, ssl, error_log, start, fails, restart, httpd, centos
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