Slow Mount Times and Upgrade Failures During SharePoint 2013 to 2016 Migration

Hi everyone,

I'm seeking advice on some challenges I'm facing while migrating a SharePoint 2013 farm to SharePoint 2016.

Initial Setup:

• Successfully configured a SharePoint 2016 farm with attached databases.
• Mounted content databases from SharePoint 2013 and upgraded them.
• Upgraded sites were accessible and browsable.
• Renamed site collections with a different domain (all worked well).

Final Cutover Issues:

• Restored all content databases from SharePoint 2013 to the 2016 SQL instance.
• Dismounted existing databases and attempted remounting from the new SQL server.
• Mounting times are significantly longer than expected:
  • 600MB database took 45 minutes (larger databases expected to take longer).
• Upgrade status shows "failed" for all content databases after remounting.
• Site collections appear to be missing or not listed properly.
• Remounting seems to overwrite permissions and undo site collection renaming.

Questions:

• Are these slow mount times normal, especially for small databases?
• What could be causing the upgrade failures after remounting?
• How can I ensure proper site collection listing and prevent permission overwrites?
• Is it safe to dismount and remount failed databases again for another attempt?
• Given the different web application URLs in the old and new environments, are there any specific considerations for renaming site collections?

Any insights or suggestions on troubleshooting these issues would be greatly appreciated.

Thanks,

Pradeep