Atlas: diagnose chunk imbalance and jumbo chunks

Shard imbalance

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

Diagnose

Run sh.status() (or the Atlas sharding views). Look at chunk counts per shard and the balancer state. Chunks that cannot split or move are flagged jumbo.

Confirm

• Jumbo chunks: usually a low-cardinality shard key value that outgrew the chunk size. They cannot be moved or split automatically.
• Balancer off or throttled: chunks pile up on one shard.
• Monotonic shard key: all new writes land on the max-key chunk of one shard.

Fix

• Jumbo chunks need manual intervention: clearJumboFlag (requires atlasAdmin) after splitting manually, or reshard the collection with a better key.
• Turn the balancer on and let it drain; for big imbalances it takes hours, not minutes.
• A monotonic key is a design bug: plan a reshard with a hashed or compound key.

Verify

Chunk counts converge across shards, no jumbo flags remain, and write load spreads evenly in the metrics.

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