Migrating database, and lowering n_val

[systream]

One of our Riak clusters runs on 2.x, which needs to be migrated to 3.x.
The problem is that we want to decrease n_val. What is the best way to migrate/copy a database with different n_val values.
The service cannot be stopped, and has a few Terra bytes per node.

We have some ideas on how to handle it, but I thought it would be worth asking a question, is there a good practice, tool?

[martinsumner]

There is a process for doing this with nextgenrepl, but are you using the new (tictac) AAE?

With riak_repl, increasing n_val I think is possible - but decreasing n_val is potentially very tricky. With riak_repl full-sync it is vnode-to-vnode replication not cluster-to-cluster, and so when your new cluster's vnodes are going to have a subset of the data I'm not sure how this will behave.

In the past I've done some bespoke stuff outside of riak_repl for migrations. The issue with bespoke answers tends to be the final stage of validating that clusters are in sync after migration.