Non-Disruptive Upgrades Overview
All MDS switches support non-disruptive upgrades and downgrade subject to limitations listed in the MDS release notes.
On a 95xx or 97xx Series MDS with dual supervisors, during the firmware upgrade, the new code is loaded on the standby supervisor. A switchover then occurs in order to make the supervisor that runs the new code active. The code is then loaded on the previously active supervisor, and it becomes the new standby supervisor. The data plane continues to pass fibre channel traffic. The modules then non-disruptively start at the lowest numbered module and proceed to the highest.
On a 91xx, 92xx, or 93xx Series MDS, which has only one supervisor, the supervisor (control plane) is reloaded. The data plane continues to pass fibre channel traffic.
Reference:
https://www.cisco.com/c/en/us/support/docs/storage-networking/mds-9000-nx-os-san-os- software/118952-technote-mds9k-00.html