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Merge pull request #48298 from k-morozov/docs/background_pool_size

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upadte doc for background_pool_size
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Rich Raposa 2023-03-31 13:06:45 -06:00 committed by GitHub
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docs/en/operations/server-configuration-parameters/settings.md
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@ -1047,7 +1047,7 @@ Default value: `0`.
Sets the number of threads performing background merges and mutations for tables with MergeTree engines. This setting is also could be applied at server startup from the `default` profile 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.
Before changing it, please also take a look at related MergeTree settings, such as `number_of_free_entries_in_pool_to_lower_max_size_of_merge` and `number_of_free_entries_in_pool_to_execute_mutation`.
Before changing it, please also take a look at related MergeTree settings, such as [number_of_free_entries_in_pool_to_lower_max_size_of_merge](../../operations/settings/merge-tree-settings.md#number-of-free-entries-in-pool-to-lower-max-size-of-merge) and [number_of_free_entries_in_pool_to_execute_mutation](../../operations/settings/merge-tree-settings.md#number-of-free-entries-in-pool-to-execute-mutation).
Possible values:

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docs/en/operations/settings/merge-tree-settings.md
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@ -553,6 +553,32 @@ Default value: 8192
Merge reads rows from parts in blocks of `merge_max_block_size` rows, then merges and writes the result into a new part. The read block is placed in RAM, so `merge_max_block_size` affects the size of the RAM required for the merge. Thus, merges can consume a large amount of RAM for tables with very wide rows (if the average row size is 100kb, then when merging 10 parts, (100kb * 10 * 8192) = ~ 8GB of RAM). By decreasing `merge_max_block_size`, you can reduce the amount of RAM required for a merge but slow down a merge.
## number_of_free_entries_in_pool_to_lower_max_size_of_merge {#number-of-free-entries-in-pool-to-lower-max-size-of-merge}
When there is less than specified number of free entries in pool (or replicated queue), start to lower maximum size of merge to process (or to put in queue).
This is to allow small merges to process - not filling the pool with long running merges.
Possible values:
- Any positive integer.
Default value: 8
## number_of_free_entries_in_pool_to_execute_mutation {#number-of-free-entries-in-pool-to-execute-mutation}
When there is less than specified number of free entries in pool, do not execute part mutations.
This is to leave free threads for regular merges and avoid "Too many parts".
Possible values:
- Any positive integer.
Default value: 20
**Usage**
The value of the `number_of_free_entries_in_pool_to_execute_mutation` setting should be less than the value of the [background_pool_size](/docs/en/operations/server-configuration-parameters/settings#background_pool_size) * [background_pool_size](/docs/en/operations/server-configuration-parameters/settings#background_merges_mutations_concurrency_ratio). Otherwise, ClickHouse throws an exception.
## max_part_loading_threads {#max-part-loading-threads}
The maximum number of threads that read parts when ClickHouse starts.