When working with MergeTree tables in ClickHouse, data distribution is crucial and is primarily based on the storage policy implemented during table creation. The storage policy determines how the data is distributed across the underlying disks and volumes, ensuring efficient load balancing and optimal data placement.
Let’s consider a scenario where you have a storage policy using an additional volume and underlying using three disks. In that case, By default, ClickHouse uses the round_robin policy for load balancing the rows; and data distribution happens between the disks in a round-robin manner. Each insert (or merge) creates a part on the next disk in the volume.
Alternatively, ClickHouse offers another load-balancing policy called “least_used.” This policy distributes new data to the disk with the least written data. By utilizing the “least_used” policy, ClickHouse ensures that the disk with the most available space receives the newest data, enabling optimal storage utilization.
Although ClickHouse defaults to the “round_robin” policy, you can specify the “least_used” policy by adding the
<load_balancing>least_used</load_balancing> line to your storage configuration file, allowing you to customize data distribution according to your requirements.
To gain further insight into your setup and understand how data is distributed, you can execute the following queries –