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Thayne
Asked: 2021-06-08 09:12:29 +0800 CST

Can't write to /tmp in systemd service with PrivateTmp=true

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My apache2 service has PrivateTmp=true. When the service first starts it works fine, but after a few days, writing to /tmp fails with "no such file or directory". To debug I've tried running nsenter -t <apache-pid> -m bash and I've confirmed that /tmp exists, but mkdir /tmp/test fails with "no such file or directory". I'd rather not remove the PrivateTmp=true directive.

If I restart the service it starts working again.

The mount line for /tmp inside says it is mounted to /dev/nvme0n1 which seems odd to me, but that is the case both when /tmp is working immediately after starting the service and when it is not writable.

Anyone know why /tmp suddenly becomes unwritable?

linux systemd systemd-service

2 Answers

  1. Best Answer

    I found the problem.

    I had tmpreaper enabled and configured to clean up old files and directories under /tmp. I didn't have an exclude rule for /tmp/systemd-private-*, so tmpreaper was deleting the private tmp directory for apache2.

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  2. As I understand it PrivateTmp=true forbids exactly what you tried when you were debugging. The daemon will create it's own subdirectory and change it's namespace accordingly.

    As your problem only occurs after some time I have the following advice: Make sure the application cleans up and does not store large files in the virtual /tmp directory. As far as I know this directory is using RAM, no persistent file system. You have limited space here.

    You might want to log the /tmp directories size for a while. If it keeps growing that's the issue.

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