Resharding.

the two-minute refresher

• hash % 4 and hash % 5 disagree for most keys: going from 4 to 5 partitions moves about 80% of the data.
• That migration costs network, disk and risk, all while serving traffic.
• Consistent hashing cuts it to about 1/(N+1) of the keys.
• Plan resharding from day one: it is far harder on a live system.