A non-disruptive upgrade (NDU) is an update to software or hardware that does not interrupt access to data or system service. An NDU does not require the system to be rebooted when the upgrade process completes.
There may be variation in the way vendors characterize non-disruptive upgrades. For instance, some vendors may tolerate performance degradation while an upgrade takes place, while others may define an NDU as an update that has no impact on performance. Non-disruptive upgrades can be achieved in different ways. For instance, a data storage system with two controllers might divert I/O from one controller to the other controller while the upgrade takes place. A storage system with multiple controllers might do a rolling upgrade, one node at a time. Some storage vendors may require an external tool such as multi-pathing software for a product to achieve a non-disruptive upgrade.