openQA actually has a feature for doing this itself (at least, it cleans up ISOs). openQA has an async job scheduler called 'Gru' (it's a simplified version of a Mojolicious plugin called 'Minion'...get it? Gru is in charge of the minions...) and one of the default jobs is limit_assets(), which is scheduled to run each time a job is created. It throws away old ISOs, more or less, I think.

https://github.com/os-autoinst/openQA/blob/master/lib/OpenQA/Schema/Result/Assets.pm#L112

I enabled openqa-gru on BOS; it should clean up ISOs for us. I set a size limit of 300GiB (this is set in the database, there appears to be no UI for it). We can keep an eye on it and see if it actually works. To debug it you actually have to run gru at a console, though, it seems like gru output doesn't get logged anywhere when it's running as a service.

openqa-gru has been doing its job fine.

This issue has been migrated to Fedora Forge:
https://forge.fedoraproject.org/quality/fedora_openqa/issues/11

Please continue any further discussion there.