Error when mounting FSx for ONTAP file system - mount.nfs: Failed to resolve server ... Name or service not known ... Operation already in progress

I will suggest reviewing the inbound rules specified in your VPC security group, and make sure that the security group associated with your file system has the corresponding inbound rules. https://docs.aws.amazon.com/fsx/latest/ONTAPGuide/limit-access-security-groups.html#create-security-group

Thank you @AWS-User-1542197 for the helpful clue. I had a well configured security group with all the right rules, with consistent VPC and subnet, but the security group needed to be "associated with the file system" in the right way.

Navigating to FSx > File Systems > [my file system] ... at the bottom of the page, under "Network interface", I found two elastic network interfaces associated with my file system. Accessing each one of those ENIs (EC2 > Network interfaces > [eni-...], I went to Actions > Change security groups. I removed the "default" security group that was there and added my own security group with all the right inbound and outbound rules.

For the mount command over on my EC2 instance, I could not get the DNS name to work (error: "Name or service not known"). Presumably a DNS lookup problem. Ping didn't work either. The NFS DNS name and IP address were found here: FSx > File Systems > [my file system] > Storage virtual machines > [my SVM] > Endpoints > [my NFS DNS name] and [my NFS IP address]

$ sudo mount -t nfs -o nfsvers=4.1 svm-abcdef1234567890c.fs-012345abcdef6789b.fsx.us-east-2.amazonaws.com:/vol1 /fsxN $ mount.nfs: Failed to resolve server svm-abcdef1234567890c.fs-012345abcdef6789b.fsx.us-east-2.amazonaws.com: Name or service not known

However, using the NFS IP address worked.

$ sudo mount -t nfs -o nfsvers=4.1 [my NFS IP address]:/vol1 /fsxN $

"Ensure that there's an Amazon EFS mount target in the same Availability Zone as the Amazon EC2 instance" https://docs.aws.amazon.com/efs/latest/ug/troubleshooting-efs-mounting.html#mount-fails-dns-name