Atlas: diagnose chunk imbalance and jumbo chunks

TL;DR: One shard is hot while others idle. Check chunk distribution, find jumbo chunks, and fix the balancer or the shard key. Shard imbalance Symptom: uneven disk or CPU across shards; the balancer never catches up. Confirm - Jumbo chunks: usually a low-cardinality shard key value that outgrew the chunk size.

The error

uneven disk or CPU across shards; the balancer never catches up. Confirm - Jumbo chunks: usually a low-cardinality shard key value that outgrew the chunk size.

The fix

TL;DR: One shard is hot while others idle. Check chunk distribution, find jumbo chunks, and fix the balancer or the shard key. Shard imbalance Symptom: uneven disk or CPU across shards; the balancer never catches up. Confirm - Jumbo chunks: usually a low-cardinality shard key value that outgrew the chunk size.

The error

uneven disk or CPU across shards; the balancer never catches up.

The fix

1. Jumbo chunks: usually a low-cardinality shard key value that outgrew the chunk size. They cannot be moved or split automatically.
2. Balancer off or throttled: chunks pile up on one shard.
3. Monotonic shard key - all new writes land on the max-key chunk of one shard.
4. Jumbo chunks need manual intervention: clearJumboFlag (requires atlasAdmin) after splitting manually, or reshard the collection with a better key.
5. Turn the balancer on and let it drain; for big imbalances it takes hours, not minutes.
6. A monotonic key is a design bug: plan a reshard with a hashed or compound key.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: Atlas.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

• Not pinned to a specific version; follows current Atlas behavior.

Sharding balancer, chunk management, and jumbo chunk handling per https://www.mongodb.com/docs/manual/sharding/, verified 2026-09-26.