**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

```text
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.