Documentation link: https://docs.redhat.com/en/documentation/openshift_container_platform/4.16/html/machine_configuration/machine-config-index#about-machine-config-operator_machine-config-overview

We have the following text into the important yellow box:
"When the MCO detects changes to the /etc/containers/registries.conf file, such as adding or editing an ImageDigestMirrorSet, ImageTagMirrorSet, or ImageContentSourcePolicy object, it drains the corresponding nodes, applies the changes, and uncordons the nodes. The node drain does not happen for the following changes:The addition of a registry with the pull-from-mirror = "digest-only" parameter set for each mirror.The addition of a mirror with the pull-from-mirror = "digest-only" parameter set in a registry.The addition of items to the unqualified-search-registries list." 

This text may be modified a bit more to provide better clarity. For example, it doesn't provide information about what will happen when removing an ImageDigestMirrorSet, ImageTagMirrorSet or ImageContentSourcePolicy objects.

Also, when adding an ImageDigestMirrorSet, it won't drain the node.

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