source parts size is greater than the current maximum

Symptom

I see messages like: source parts size (...) is greater than the current maximum (...) in the logs and/or inside system.replication_queue

Cause

Usually that means that there are already few big merges running. You can see the running merges using the query:

SELECT * FROM system.merges

That logic is needed to prevent picking a log of huge merges simultaneously (otherwise they will take all available slots and ClickHouse® will not be able to do smaller merges, which usually are important for keeping the number of parts stable).

Action

It is normal to see those messages on some stale replicas. And it should be resolved automatically after some time. So just wait & monitor system.merges & system.replication_queue tables, it should be resolved by it’s own.

If it happens often or don’t resolves by it’s own during some longer period of time, it could be caused by:

1. increased insert pressure
2. disk issues / high load (it works slow, not enough space etc.)
3. high CPU load (not enough CPU power to catch up with merges)
4. issue with table schemas leading to high merges pressure (high / increased number of tables / partitions / etc.)

Start from checking dmesg / system journals / ClickHouse monitoring to find the anomalies.