Another caveat is that when a shard is split because it has reached its max size, the resulting new 2 shards each get half of the throughput the initial shard h

Another caveat is that when a shard is split because it has reached its max size, the resulting new 2 shards each get half of the throughput the initial shard had, and not a proportion of the total throughput. So say you have 4000 write capacity and 0 reads (for simplicity). DynamoDb will allocate 4 shards for it, each getting 1000 writes. Now say shard 2 gets too big, and goes above 10GB. DynamoDb will split it in two. Now you have 5 shards, but they won't all get 4000 / 5 = 800 writes. Instead, the original shard 1 3 and 4 will each still have 1000, and shard 2a and 2b will have 500 each. That's because when dynamodb splits a shard, it redistribute the throughput of its parent to the shards.