The lack of hardlinking for composes could be pretty anoying.
An alternative to moving /compose off might be to move off:
/work (failed build logs from the past week/in progress builds) /repos (all the buildroot repos) /scratch (all scratch builds)
I think that would move a similar amount of space (~15TB).
However, we would definitely need an outage for that. No builds could be going when we move things. ;(
I've dropped an email to koji developers for advice...
So, stay tuned.
At least right now we have about ~11TB free, so hopefully we can last a while longer before we have to do anything.
kevin