If site2 goes down, including the cluster master, the surviving search head(s) can still search site1, even if the cluster master if offline.
However, they will search using the "last known" information, which might not be good.
So I would do 2 things:
First, I would set the site1 search head to site1, not site0. And the site2 search head to site2, not site0. Why? Because I want the "last known" information to always be for the local site. That way, if the other site goes down, the search head will still be able to search. Using site0 means that the "last known" information could contain indexers/buckets from any site - not just the search head's local site.
Second, I would have a backup cluster master available, on site1. If the cluster master goes down, I want to start the backup cluster master as soon as possible. This will keep the cluster up to date for both the peers and the search heads. This is particularly important for longer outages.
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