We are seeing the same behavior in our 7 node search head cluster. All hardware is identical. Here are a few things that we've noticed. Nothing proven over time. Just observations at this point.
running transfer shcluster-captain seems to kick the scheduler in a way where it temporarily redistributes scheduled searches to other nodes in the cluster.
After a few minutes, the same node begins running all scheduled searches again.
There is some ordering in play our search cluster nodes are named xyz001- xyz007, so the last digit is the only difference.
a. if xyz006 is the captain, xyz007 ends up running all of the scheduled searches.
b. if we transfer captaincy to xyz001, xyz002 runs the majority of the scheduled searches, then xyz003 picks up some of searches for about 5 to 10 minutes. Then xyz007 starts running all of the scheduled searches.
We recently upgraded to a patched version of 6.3.3 (6.3.3.2) from 6.3.1, so these are our initial observations that may or may not be helpful.
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