A Non-Disruptive Upgrade (NDU) on a Dell PowerStore system is meticulously sequenced to ensure continuous data availability. The process always begins with the standby (peer) node. While the peer node is being upgraded, the primary (active) node continues to manage all I/O operations, preventing any service interruption. After the peer node's upgrade is successfully completed and it has rebooted, a controlled failover is initiated. The newly upgraded peer node takes over as the primary, and the original primary node then undergoes the same upgrade process. This methodical, sequential upgrade ensures that one node remains operational at all times.

A. The primary (active) node: Upgrading the active node first would interrupt I/O service, directly contradicting the "non-disruptive" nature of the procedure.

C. Both nodes simultaneously: This would cause a complete system outage, as both controllers would be offline, making it a disruptive upgrade.

D. The node with higher I/O: The node with higher I/O is, by definition, the primary (active) node. Upgrading it first would cause a service disruption.

1. Dell PowerStore: Software Upgrade Guide (Version 3.6.x)

Section: "Upgrade Process Overview"

Content: The documentation explicitly states

"The NDU process starts by upgrading the software on the peer node of the appliance. While the peer node is being upgraded

the primary node continues to service I/O to the appliance." This confirms the peer/standby node is upgraded first.

2. Dell PowerStore: High Availability (White Paper)

Section: "Non-Disruptive Upgrades (NDUs)"

Content: This paper details the architecture enabling NDUs. It describes how the system leverages its redundant

active-active architecture to perform upgrades. The process involves placing one node in service mode (the peer node) for the upgrade

while the other continues to operate. It states

"The upgrade process is performed one node at a time

starting with the peer node."