The rolling restart essentially runs a "splunk offline" on each indexer one-by-one. An "offline" of an indexer cluster slave is a controlled shutdown, where all buckets that it is assigned as primary for are transitioned to another slave, and buckets are replicated as needed to maintain the rep factor, or made searchable as needed to maintain the search factor. This is most of the fixup tasks.
With that being said, and in particular for large clusters, a rolling restart can be quite traumatic, especially if you are glued to the Cluster Master console. The cluster will eventually recover most of the time, and so its probably best to kick of the restart, make sure it actually took, and then give it 5 minutes before checking in.
Please let me know if this answers your question! 😄
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