I think we can build the index in codfw and the future labs replica much quicker than a dump.
updateOneSearchIndexConfig basically already does everything we need. It even takes the connection to read from (via an Elastica\Index object) and the connection to write to (via a CirrusSearch\Connection) independently.
There is a hitch: to get a consistent index without having to manually reindex specific time periods, the plan so far has been:
# Create mappings for all wiki's in codfw
# deploy operations/mediawiki-config to also send writes to codfw for one index
# Watch things for a bit
# turn on a few more. turn on the a few more. turn on the rest.
# Copy/rebuilt into the same index accepting writes
updateOneSearchIndexConfig currently always creates a new index, so it isn't directly usable. Maybe a flag? maybe shared code in a new maint class?
Additionally by having the reindexing contained to a single process we could use standard tools like trickle to rate limit our WAN traffic.