It's a brand new 3 tier farm with Index Partition 0 mirrored on both of my Web servers. I noticed a yellow warning icon on one of the index components in central admin. I ran Get-SPEnterpriseSearchStatus -Text and it shows that message in the thread title,"State: Degraded. Index out of sync - catching up". It's been like this for at least 24 hours so I assume something is wrong. So far I have tried:
Resetting the search index.
Removing the degraded index component, activating the topology. Adding the search index component again and activating the topology.
Resetting the index and performing a full crawl
So far none of that has worked. I'm not seeing anything that stands out in the degraded server event log either.
Edit; I've done some more testing and can safely say that the Index partition is healthy on a single server, as soon as the same partition is added to another server, the new servers component shows degraded, forever.
Any suggestions?