SQL Server on EC2 and FSx maintenance

Short answer: There will be no database interruption for SQL Server Always On deployments in the event of file system failover/failback as long as you're using continuously available (CA) shares on Amazon FSx multi-AZ file systems. https://docs.aws.amazon.com/fsx/latest/WindowsGuide/sql-server.html

Details: With file shares configured as CA on multi-AZ file systems, server applications like SQL Server will get uninterrupted access to the file share even in the event of a failover/failback (by transparently reconnecting to another file server node without interruption of the server applications). These server applications expect their storage to reliable and always available and they do not generally handle IO errors or unexpected closures of handles very well. If the server application cannot access its storage this often leads to databases going offline or virtual machines stopping or crashing because they can no longer write to their disk. This is why Amazon FSx supports SMB CA shares (aka SMB Transparent Failover) as one of the key features on multi-AZ file systems.