Symptoms
Attempting to create a file directory or restarting a service reports the following error
[root@test ~]# mkdir -v /tmp/application_dir/
mkdir: cannot create directory ‘/tmp/application_dir/’: Too many links
[root@test ~]# journalctl -u pdns.service
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Jul 24 04:44:53 test.88.host systemd[27938]: Failed at step NAMESPACE spawning /usr/sbin/pdns_server: Too many links
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Description
The error message "Too many links" can be a little vague if no additional context is provided. However, this message is generally provided when the FileSystem has reached it's maximum hard-links limit.
For EXT3 partitions, this limit will be 32,000 and for EXT4 systems, this is doubled at 65,000 with the option to enable "dir_nlink" as a FileSystem feature to allow for unlimited hard-links.
When these Filesystem limits are reached, services that require creating files in the directory that is full will fail as well as any other operations.
This can be most commonly seen in the "/tmp" partition which is usually where applications and software create files and directories.
Workaround
If you find that an application has created this many subdirectories within "/tmp" or any other location in your server, you will first need to delete the subdirectories to free up some space and then you will need to address the issues with the application not removing unneeded files.
Here is a similar article with the usage of Linux commands, among them the "rm" command that allows you to remove files and directories.