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Here is what I did, and what happened.

For info - we need the "+3 thing" (i.e. 6 replicas in total) as the datacentres are a long way apart (100+ miles) and have a lower capacity link between them, and I want each centre to be self sufficient.

The new data centre isn't yet on line, so I created the new rings as detailed above - i.e. in one move I upped the replica count from 3 to 6 and also changed the weighting of the previously zero weighted disks on the new centre to be all the same as the ones on the initial datacentre.

I then pushed out these rings to only the new data centres. Yes this would result in some data getting pushed back and forth, but not very much - as the balance is 0.01 or thereabouts.

The 3 servers at the new datacentre, when running a dispersion report gave only a 50% health and couldn't find half the copies - which was expected - but the original centre reported that all was ok.

2 weeks later the new datacentre have filled their replicas and are reporting a health of around 99% (something keep getting moved to the different places).

This morning I replaced the rings on the original datacentre as well and after a bit of disk churning, the whole system is now reporting 6 replicas and 100% dispersion ok.

Job done.

I can now start using the new datacentre, 2 months ahead of schedule. Now if only that would mean I could put my feet up for two months...