Slow Data Replication of ClickHouse Table Replicas and a Large Number of Fetch Requests in the Queue

Symptom

The execution result of the following SQL statement shows that the data synchronization between replicas is slow:

select table,absolute_delay, queue_size, inserts_in_queue, merges_in_queue from system.replicas where absolute_delay > 0 order by absolute_delay desc limit 10

Cause Analysis

Service data is inserted dozens of times per second, and the fetch queue is stacked according to the replica synchronization logic of ClickHouse.

Solution

Modify the background_fetches_pool_size configuration item on Manager.