The balancer could use a load assessment for deciding what chunk should be balanced where.

Any combination of the following:

• Most efficient working set
• Least faults
• Least writes
• Least network traffic
• Fastest query response time
• Lowest CPU usage
• Least memory used for sorting

Policy 1:

1. Find most active chunk by query usage (counts + size)
2. Find shard with most efficient working set (least faults)
3. Do move

Policy 2:

1. Find largest active chunk (in last X minutes)
2. Find shard with least number of chunks
3. Do move