It's pretty opaque. My impression is that each shard (storage backend quorum) gets a roughly equal share of capacity you pay for, and items in the same partition tend to live on the same shard to keep range queries small (and local indices require one-shard partitions). They've made improvements in loaning cold shards' unused capacity to hot shards, but they still recommend avoiding hot partitions and keeping load roughly even.
erik_seaberg|6 years ago