SQL Server – Resolving DB Out of Sync on Secondary Node HAG

Its very difficult to reverse engineer and interpret SQL T-log records. As mentioned there are 3rd party tools like APEX SQL Log , RedGate's LOG Rescue, Log Explorer

Also, there is an undocumented command fn_dblog that can be used as below, but you wont have the flexibility like 3rd party tools.

SELECT 
    [Previous LSN],
    [Current LSN],
    [Operation],
    [AllocUnitName],
    [Page ID],[SPID],
    [Begin Time], 
    [Transaction Name],
    [Lock Information] 

FROM ::fn_dblog(null, null)
GO

Paul Randal has described how to read from T-log with excellent examples -- Using fn_dblog, fn_dump_dblog, and restoring with STOPBEFOREMARK to an LSN .

The fastest way would be to create a cluster an join your current stand alone server to the cluster as a new node. Set the database to full recovery model, take a full backup and a transaction log backup and restore them to the instance on the other node with norecovery.

If you have a good full backup (most recent), it would be possible to set the recovery model to full, take a differential and then take a log backup using the last full from the simple recovery model as your base. The differential should bridge the lsn gap.

To answer your questions:

1. I'm not sure why you're switching the database into single_user except that you want no changes to be made. If you add the server as a node as I've set forth, this won't be an issue. Otherwise, yes, you'll need downtime to migrate the database. Your other option would be to set the recovery model to full, get it in full, setup mirroring to the new instance on the cluster and then cut over during a downtime which would be much less time.

2. That's entirely possible to do, but in your initial question you said you didn't want to do this.

3. You must restore at least one log or differential to bring the lsn gap as close as possible.

4. I would not use the wizard, but the join only means you've staged a database that is currently in norecovery and it only needs to join the AOAG. The wizard will want to pre-stage the databases for you and ask you for shared locations, etc. Like I said it's best to pre-stage and not use the wizard for the best flexibility.