Changes done so far:
- signal to route concurrency request up to the stepQueue
- pretty hacky, no status reporting yet
- stepQueue upsizing (confirmed that it works, no intermediary commit)
Stuck at: stepQueue downsizing
- idea was to have the stepQueue signal to the activated step that it
should suspend
- ideally, we'd just kill everything associated with the step and track
it as restartable
- the driver model doesn't allow for that though within an attempt
- would need to start a separate run for the step
- less perfect: tell the downsized steps to stop copying, but leave
all zfs sends + rpc conns open
- - doesn't give back the resoures that a step aquired before
being selected as downsize victim (open zfs processe + TCP conn,
some memory in the pipe)
- - looks weird to user if the ps aux
- + re-waking a step is easy: just tell it to proceed with copying
- (the impl would likely pass a check function down into Step.do
and have it check that functino periodically. the suspend should
be acknowledged, and stepQueue should only remove the step from
the active queue _after_ that step has acknowledged that is
suspended)
