Nutanix Files is a software-defined, scale-out file storage solution that provides a repository for unstructured data, such as home directories, user profiles, departmental shares, application logs, backups, and archives.

Nutanix Files comprises of 3 x File Server virtual machines which forms a highly-available file-serving cluster atop your existing AOS Cluster.

As with any business critical application, it is vital that software upgrades be as non-disruptive as possible, Nutanix offers the same “ONE-CLICK” simplicity for Upgrading your Nutanix Files Cluster.

How to plan Nutanix Files Upgrade:

• Always refer Nutanix Files x.x.x.x Release Notes before a new deployment or an upgrade.
• Always refer Software Product Interoperability page for AOS and Nutanix Files recommended interoperability. This is critical for a new deployment or upgrade.
• Always refer Upgrade Paths page before Nutanix Files upgrade. This provides you details about target release version and if they are supported for upgrade or not.

How to Upgrade Nutanix Files:

1. Login to Prism Element
2. Settings Menu > Upgrade Software
3. In the Upgrade Software window, select the File Server tab.
4. Download the Files version you want to upgrade to
5. Perform the Upgrade 

What happens when you perform Nutanix Files Upgrade:

• First, the pre-upgrade checks will run to make sure that the cluster is able to be upgraded. If any of the pre-upgrade checks fail, you will see information about this in Prism and the actual File Server upgrade will not start. Users will have to click "Back to Versions" and start the upgrade again after the issue reported by the pre-checks is resolved. To see the full list of pre-checks and their related KB articles, check out KB-6524.
• Once the File Server upgrade beings, each File Server VM is upgraded one-at-a-time onto the new Nutanix Files version. While an FSVM is down for the upgrade, users connected to shares hosted by this node may experience a loss of connectivity for a duration of roughly 20-30 seconds. After this short period, another FSVM will pick-up on hosting those shares, and users will regain access to their files.
• After each FSVM completes its reboot onto the new version of Nutanix Files, the upgrade will make sure that it can once again host shares before starting to upgrade the next FSVM.

Expected Behaviour during Nutanix Files Upgrade

During Nutanix Files upgrade process the FSVMs utilise the HA process where the External IP addresses and share ownership gets migrated to another FSVM.

During the HA process, it could take 30-60 second for this migration to complete. Users will not need re-authenticate as the file handles are are preserved. This will occur in a rolling fashion for one FSVM at a time. The client felt impact should only be delay or pause and not a drop. 

You can follow the official Nutanix Documentation and KBs before upgrading your Nutanix Files Cluster:

Nutanix Files - Deployment and Upgrade FAQ

AOS and Nutanix Files Pre-Upgrade Checks

Installing or Upgrading Nutanix Files