Monitoring Mirroring

Since Mirroring is now deprecated in favour of Always On, I think it is a minority sport these days

But it works perfectly well, especially if you haven't got Always On.  In a perfect world, we would all upgrade to the latest version as soon as it comes out, and take advantage of all the whizzy new features.  But we don't, of course

Anyway, monitoring mirroring.  A bit of a tongue twister, that.

Assuming you have set up mirroring already, right click on the database you want to monitor, select Tasks, and choose Launch Database Mirroring Monitor

Click on the Warnings tab.  Do this for each of the mirrored databases

Initially, it doesn't monitor anything.  Tucked away in the bottom right of the screen is a button marked Set Thresholds.  Click on it and tick all the boxes

Microsoft sets these defaults - 
Unsent log exceeds 1KB
Unrestored log exceeds 1KB
Oldest unsent transaction exceeds 1 minute
Mirror commit overhead exceeds 1millisecond

I would recommend starting with these and seeing how it goes.  You will almost certainly end up upping those defaults, but it probably isn't my place to tell you what to do

‌Remember to go to the SQL Agent jobs - find Database Mirroring Monitor Job - and set it to Notify you if it finds a problem.  

Be sure to set up the alerts and the Agent Job at both servers, so that it continues to alert you in the event of a failover.