Migrating VLDB to AlwaysON AG I have been using AlwaysON AG for a long time now and currently have about 10TB of data across 120 databases and 3 AG groups for any application that is on SQL 2012 with great success. Each AG group is running on patch level 11.0.5058.0 with 2 synchronous replica(on different SANS) in Primary Data center and 1 ASYNC replica in DR. Migration has been a non-issue because none of the databases were substantial enough that I could not fit into my maintenance window which is 12-4AM on SAT morning. 

My issue is that my last application to migrate to 2012 includes a 4TB TDE encrypted databases database which is about 10x larger than any of the previous ones I have migrated. The database takes 4 hours to backup after tuning extensively(I hate TDE!!)

The restore to the primary replica is instant because of seeding incremental but the issue comes from having to backup the database before adding to the availability group. 4 hours is my exact outage window and I can't get any more. 

My plan to migrate application is to -

First Outage Window


Restore Database from 2008 to 2012 Primary Replica
Change application ARECORD(or cname not sure which) to Primary replica 
Run database on single node until next outage window


Week Later :


Add database to availability group
Change ARECORD/CNAME to listener 


What I don't like about this is I am going an entire week with 1 node instead of 3 which is worrisome. If anyone else has any better ideas on how to accomplish this I would love to hear from you or any type of comment from people who have worked with VLDB in availability groups and what you like/hate/loved about doing it. I am trying to go all in on this software and have loved it so far but getting worried when it comes to the VLDB migration.
The requirement that a database have a full backup is only enforced by the Wizard. If you just Add the database after the Availability group is already created you can restore the same backup on both the Primary and Secondary and then add the database to the Availability Group. To save time during the maintenance window we usually restore Full backups ahead of time and take a differential backup during the maintenance window.


Take Full backup of the database being moved.
Restore the full backup on the New primary WITH NORECOVERY this ensures the database isn't used until the secondary is ready.
Restore the same full backup on the new secondary WITH NORECOVERY this is needed to add them to the AG.
Restore the primary database WITH RECOVERY. RESTORE DATABASE [MyBigDB] WITH RECOVERY;
On the Primary instance navigate to AlwaysOn High Availability > Availability Groups > MyAG > Right-Click on Availability Databases > Add Database
Select your database 
Select Join only 
Connect to the secondary instance 
All the validation should be skipped 
Verify everything and click finish or Save the script. 
Verify syncronization is happening by viewing the Availability Group Dashboard