M3DB Subclustered Placement Algorithm

Uber has evolved its M3DB database architecture by introducing a subclustered placement algorithm. This addresses operational reliability and maintenance overhead issues encountered with traditional sharded placement in large clusters. The subclustered approach partitions nodes into self-contained groups, each owning a distinct shard space, thereby reducing the blast radius of failures and enabling greater parallelism in cluster operations. A greedy shard donation strategy is employed during scale-up to minimize intra-subcluster skew and avoid costly rebalancing operations. This enhancement improves the scalability and maintainability of M3DB deployments.