If you want to check by yourself that the pull-backup instances do their job, you can change directory to the slappart of runner0, and run ./bin/exporter (after creating your account, using the given backend_url or url): it would fill ./srv/backup/runner with data. If you then go to an import instance (runner1 or runner2) on the port 50005, you should be able to sign in the runner.
Tips :
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To find in which partition the instance has been deployed, you can open the page of this specific instance, and look for "slappartXX" on the page.
Tests :
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For the tests, please refer to the README in slapos.toolbox.git/slapos/resiliencytest