background_buffer_flush_schedule_pool_size
The maximum number of threads that will be used for performing flush operations for Buffer-engine tables in the background.background_common_pool_size
The maximum number of threads that will be used for performing a variety of operations (mostly garbage collection) for *MergeTree-engine tables in the background.background_distributed_schedule_pool_size
The maximum number of threads that will be used for executing distributed sends.background_fetches_pool_size
The maximum number of threads that will be used for fetching data parts from another replica for *MergeTree-engine tables in the background.background_message_broker_schedule_pool_size
The maximum number of threads that will be used for executing background operations for message streaming.background_move_pool_size
The maximum number of threads that will be used for moving data parts to another disk or volume for *MergeTree-engine tables in a background.background_pool_size
Sets the number of threads performing background merges and mutations for tables with MergeTree engines.- This setting could also be applied at server startup from the
defaultprofile configuration for backward compatibility at the ClickHouse server start. - You can only increase the number of threads at runtime.
- To lower the number of threads you have to restart the server.
- By adjusting this setting, you manage CPU and disk load.
Smaller pool size utilizes less CPU and disk resources, but background processes advance slower which might eventually impact query performance.
number_of_free_entries_in_pool_to_lower_max_size_of_merge.number_of_free_entries_in_pool_to_execute_mutation.number_of_free_entries_in_pool_to_execute_optimize_entire_partition