Trace Flag impact to Mirrored Databases
What (if any) impact to mirrored databases will occur after applying -t1222 and restarting the server?


Since 1222 writes deadlock information to the log, the impact to mirroring itself will be nothing. Secondary effects should also be intangible in a normal environment. I'd only be worried if, for some reason, you have so many deadlocks that the Errorlog ends up filling your drive as this is fairly benign.

Also, by default, the extended events session does track deadlock information but it may not have all of the deadlock graph given the size of the deadlock graph.