Took a quick look at this - seems like a large latency excursion on the appserver side (https://grafana.wikimedia.org/goto/_0N8q7hIk?orgId=1), which correlates with a large spike in reads to db section s6 (https://grafana.wikimedia.org/goto/mAWw372Iz?orgId=1). Seems to have recovered fairly promptly, whatever this was.

Edit: This has the feel of a thundering herd on cache miss (or I guess invalidation, given the synchronized behavior). I see a very large correlated spike in misses for a specific key group (flaggedrevs_includes_synced, though there may be others an order of magnitude down that are harder to see) in https://grafana.wikimedia.org/goto/CcgUe7hIz?orgId=1, but that of course does not imply causation.