Comment 0 for bug 1827628

It is currently not possible to use affinity/anti-affinity with the placement "limits" parameter since if you want your instance to land on a node where another instance lives you cannot rely on what placement GET/allocation_candidates would return and this could result in a no valid host. Current workaround is to unset the limits parameter.

We already have the same-ish problem for disabled computes (https://bugs.launchpad.net/nova/+bug/1805984). We could just do a more generic solution that fits both these cases.