Ref T10967. We still have double writes, so all reviewers are being written to both old and new storage. This migrates all the data in the old storage to the new storage, so both storage tables should have a complete set of data and be getting identical updates as we move forward.
After this, I can move readers over one at a time and eventually get rid of the old writes and old storage.
This loads all of the edge data into memory in a big chunk. I reached out to one install to get some more information about their data size. Ours is quite manageable and I think even large installs will probably fit into memory, but we can do this in chunks if not.
However, because the Edge table doesn't have an id column, we can't use either the RawMigrationIterator or the MigrationIterator, and would need to write a new EdgeMigrationIterator. This isn't tons of work but might not be necessary.