Resharding From 8 to 12 Nodes Online

An application shards accounts with shard = hash(account_id) mod 8 across 8 MySQL nodes. Storage is at 85 % and the team wants to go to 12 nodes with no downtime.

1. Quantify what fraction of rows move if they simply change the modulus to 12, and explain why that is a problem beyond just data volume.
2. Propose a partitioning scheme that makes this and future expansions cheap, and estimate data movement for 8 → 12 under it.
3. Outline the online migration procedure for a single moving partition.