How to move data files of the mirror database

The ideal scenario for database mirroring would be to have the principal and secondary server instances be exact clones of each other from a hardware and disk space standpoint. This would ensure that the performance on the mirror instance is the same as the principal instance. But due to multiple constraints, this is not always possible. I recently had a question on how to move a secondary data file to a different physical location on the mirror instance. The catch here is that the physical data file locations on the principal and mirror instances are not the same. Now this can be tricky if not attempted or tested in your environment. The Microsoft SQLCAT(Twitter|Blog) team has a blog post on how to move the transaction log of a mirrored database to a secondary location. We shall use the same logic for the data file as well.


  1. If the server on which the data file relocation has to be done is not the mirror server currently, then failover the mirrored database so that the server on which the OS File has to be physically moved is the current Mirror.
  2. Change to High Performance mode if running under High Safety.
  3. Execute the ALTER DATABASE statement to move the data file to the new location on the mirror instance.
  4. Pause Mirroring to prevent any automatic failover while OS File Copy is happening.
  5. Stop the Mirror instance.
  6. Copy the file across to the new location (OS File Copy).
  7. Start the Mirror Instance.
  8. Resume mirroring and switch back to High Safety if the mirroring was running under High Safety. The mirroring should continue without any issues. Failback to original server if needed.

If the data file locations are the same on the principal and mirror and the data files need to be moved to different locations on both the server instances, then this is much easier. You would need to fire the ALTER DATABASE command on the principal server instance and then move the data files to the new location after failing over the database. The new data file location would take effect only after the server instance is restarted.