[orionblastar]

When I did DB migrations, I first tested out my code on a test server that had a copy of the production database so see how well it would go and if there were any problems.

I would test out all code on a test server first before putting it on a production server.

Migrations were usually scheduled at Midnight and finished before anyone came into work. Before the Migration a backup of all data was done.

It is the only safe way to do things.

[mrits]

You risk losing data before the between the start of the migration and the end of it. Unless you disable the service. In that case you certainly don't have zero downtime.