Yep, you’ve seen so many like this one, mostly they will suggest you to stop Exchange searching services, then delete the indexing folder (located inside the database folder) and then restart the searching services.
Well, that would work if you don’t have DAG.
Again, mostly you’ll find suggestions to use the update-mailboxdatabase with -catalogonly key
Yeah, that would work if any of the DAG members have a healthy catalog. Now what if all DAG members doesn’t have it?
You’ve two options, but both are risky and I have no responsibility what so ever happens to your data.
Remove one of the copies of the database from one DAG member
Delete the database folder copy from that member’s disk
Add the database copy again to the DAG member
Database index shows crawling for few hours, then healthy
Finally, you can use this new catalog to update other DAG members
Option 2 (have downtime):
Dismount all database copies on all DAG members
Shutdown Exchange searching services (two services) on all DAG members
Delete the catalog folder from all copies on all members
Restart Exchange searching services (two services) on all DAG members
All servers will start crawling