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Before this patch the load balancer was equalizing tablet count per shard, so it achieved balance assuming that: 1) tablets have the same size 2) shards have the same capacity That can cause imbalance of utilization if shards have different capacity, which can happen in heterogenous clusters with different instance types. One of the causes for capacity difference is that larger instances run with fewer shards due to vCPUs being dedicated to IRQ handling. This makes those shards have more disk capacity, and more CPU power. After this patch, the load balancer equalizes shard's storage utilization, so it no longer assumes that shards have the same capacity. It still assummes that each tablet has equal size. So it's a middle step towards full size-aware balancing. One consequence is that to be able to balance, the load balancer need to know about every node's capacity, which is collected with the same RPC which collects load_stats for average tablet size. This is not a significant set back because migrations cannot proceed anyway if nodes are down due to barriers. We could make intra-node migration scheduling work without capacity information, but it's pointless due to above, so not implemented.