Each update may come with new firmware or updates to external resources. In general, these updates are backward compatible and applied automatically without user intervention. There are exceptions, however, for non-reversible updates. These updates involve updating a resource external to the system software in a way that is incompatible with older software releases. Once the update is applied, rolling back to previous versions will result in undefined behavior. For these updates, you will always be given an explicit option of applying them automatically during upgrade or applying them after the fact. They are therefore referred to as "deferred updates".
When applying an update to a version with incompatible version changes, you are given an option to apply these version changes as part of the update. For each version change, the benefits of applying the change are presented to you. The default is to not apply them, requiring you to return to the updates view and apply them once the system has rebooted after the update is applied. This allows you to verify that the rest of the software is functional and a rollback is not required before applying the update.
If you elect to not apply deferred updates during an update, you can return to the updates view at any point to apply the update. If deferred updates are available for the current software version, they appear as a list below the current set of available updates, with an "Apply" button to apply the updates. Deferred updates in a cluster take effect on both storage controllers simultaneously, and can only be applied while both controllers are operational. Because deferred updates are listed only for resources present on the local storage controller, in a cluster it may be the case that deferred updates are available only for resources active on the peer controller. In a cluster, it is therefore necessary to check both storage controllers to determine the availability of deferred updates.
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