Ansible fork and serial relationship?

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Hi experts,

I found one interesting thing about ansible fork and serial. But don’t know what is the exact relationship between these two values.
Following is the result of my testing:
21 VMs

  1. fork=10, serial=100%, ansible will fail with some hosts unreachable randomly.
  2. fork=15/25, serial=100%, ansible will success.
  3. fork=10, serial=10. ansible will success.

Te conclusion: seems the serial shall be less than fork, or can’t be much larger than fork.
Question:

  1. the randomly unreachable failure is a ansible bug? I foud one issue report: https://github.com/ansible/ansible/issues/18188
  2. Is there any suggestion about the fork and serial value setting? I don’t find any document about this.

Thanks,
Ruby