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> * periodic updates can overwhelm things.  Solution: remove unneeded updates,
> most scheduling data only changes when an instance does some state change.

It's not clear that periodic updates do overwhelm things, though.
Boris ran the tests.  Apparently 10k nodes updating once a minute
extend the read query by ~10% (the main problem being the read query
is abysmal in the first place).  I don't know how much of the rest of
the infrastructure was involved in his test, though (RabbitMQ,
Conductor).

There are reasonably solid reasons why we would want an alternative to
the DB backend, but I'm not sure the update rate is one of them.   If
we were going for an alternative the obvious candidate to my mind
would be something like ZooKeeper (particularly since in some setups
it's already a channel between the compute hosts and the control
server).
-- 
Ian.