Aiven for ClickHouse® 25.8 default settings

Aiven for ClickHouse® uses a managed configuration that differs from upstream ClickHouse defaults. These differences help keep services reliable, secure, and predictable in Aiven-managed environments.

The following sections list the settings where Aiven defaults differ from defaults in version 25.8, grouped by session, table, and server scope.

How to read the settings tables

Each setting includes an Aiven default value and indicates whether you can change it:

• Can be changed: No: Aiven manages the setting value. You cannot override it.
• Can be changed: Yes: Aiven sets the default value, but you can change it where the setting is configurable.

Where Aiven defaults vary by service plan or node resources, the Aiven default column shows ranges or placeholders. The Description column explains why the default differs or lists any configurable bounds, such as an allowed range or minimum value.

Values shown as ranges or placeholders, such as [3..80, depending on CPU count], {cpu_count}, or [~7% of RAM], are sized automatically based on your service plan and node resources.

note

For configurable settings and Aiven-defined limits, see Advanced parameters for Aiven for ClickHouse® and Limits and limitations.

Session settings

These settings apply to sessions and queries.

Setting	Aiven default	Can be changed	Description
allow_deprecated_error_prone_window_functions	0	No	Keeps deprecated, error-prone window functions disabled.
allow_deprecated_snowflake_conversion_functions	0	No	Keeps deprecated Snowflake conversion functions disabled.
allow_experimental_kafka_offsets_storage_in_keeper	0	No	Keeps experimental Kafka offset storage in Keeper disabled.
allow_introspection_functions	0	Yes	Keeps sensitive debugging and introspection functions disabled by default.
allow_non_default_profile	0	No	Prevents switching to unmanaged profiles.
cancel_http_readonly_queries_on_client_close	1	Yes	Cancels read-only HTTP queries when clients disconnect.
cluster_function_process_archive_on_multiple_nodes	0	Yes	Disables archive processing on multiple nodes by default.
compatibility	""	Yes	Does not force a lower compatibility profile by default.
database_replicated_allow_replicated_engine_arguments	1	No	Supports replicated database and table creation in Aiven-managed services.
distributed_ddl_entry_format_version	7	No	Pins the distributed DDL entry format to avoid inconsistencies during version upgrades.
distributed_ddl_output_mode	null_status_on_timeout	Yes	Makes distributed DDL timeout reporting more resilient to recoverable errors.
format_display_secrets_in_show_and_select	0	Yes	Avoids exposing secrets in SHOW and SELECT output by default.
http_write_exception_in_output_format	0	Yes	Keeps exceptions out of the requested output format so client errors are easier to detect.
max_autoincrement_series	1000	No	Limits automatic sequence expansion.
max_concurrent_queries_for_all_users	[100..800, depending on service size]	No	Enforces a service-wide concurrency limit.
max_http_get_redirects	10	Yes	Limits HTTP redirect following.
max_insert_threads	2	Yes	Allowed range: 1..{cpu_count}. Caps insert parallelism so a single query does not use too many shared service resources.
max_threads	{cpu_count}	Yes	Allowed range: 1..{cpu_count}. Caps general query parallelism to the node CPU count.
memory_profiler_sample_probability	0	No	Disables random memory profiler sampling.
memory_profiler_step	4194304	Yes	Minimum value: 100000. Keeps the upstream memory profiler granularity while preventing very low values.
min_free_disk_ratio_to_perform_insert	0.05	No	Keeps a disk headroom guard before accepting inserts.
output_format_json_quote_64bit_integers	1	Yes	Keeps JSON output compatible with clients that cannot safely represent 64-bit integers. This default can change in a future version.
postgresql_connection_attempt_timeout	10	Yes	Limits each PostgreSQL connection attempt.
postgresql_connection_pool_connect_timeout	10	Yes	Limits pooled PostgreSQL connection setup.
push_external_roles_in_interserver_queries	0	No	External authorization is not supported. Roles must be synchronized between nodes.
query_profiler_cpu_time_period_ns	1000000000	Yes	Minimum value: 1000000. Keeps the upstream CPU profiler sampling period while preventing sub-millisecond values.
query_profiler_real_time_period_ns	1000000000	Yes	Minimum value: 1000000. Keeps the upstream real-time profiler sampling period while preventing sub-millisecond values.
readonly	0	Yes	Controls read/write access for the session. You can change the value from 0 to 1, but not from 1 to 0.
stream_like_engine_allow_direct_select	1	Yes	Allows direct SELECT queries on stream-like engines, which can have suboptimal performance or behavior. A future release returns this default to the upstream default.
write_full_path_in_iceberg_metadata	1	Yes	Writes full Iceberg metadata paths for compatibility with mainstream ClickHouse behavior.

Replicated MergeTree table settings

These settings apply to the ReplicatedMergeTree table engine family. On the Aiven platform, MergeTree engines are remapped to their ReplicatedMergeTree variants. See Supported table engines.

Setting	Aiven default	Can be changed	Description
allow_remote_fs_zero_copy_replication	1	No	Enables zero-copy replication for remote filesystem storage. Aiven manages this storage behavior.
disable_detach_partition_for_zero_copy_replication	0	No	Keeps DETACH PARTITION available for zero-copy replicated tables.
disable_fetch_partition_for_zero_copy_replication	0	No	Keeps FETCH PARTITION available for zero-copy replicated tables.
disable_freeze_partition_for_zero_copy_replication	0	No	Keeps FREEZE PARTITION available for zero-copy replicated tables.
enable_max_bytes_limit_for_min_age_to_force_merge	1	No	Enforces the byte-size limit when age-based forced merges are considered. This helps prevent creation of very large parts.
finished_mutations_to_keep	10	Yes	Limits retained finished mutation metadata so ZooKeeper or Keeper state does not grow unnecessarily.
max_parts_in_total	10000	Yes	Allowed range: 100..50000. Sets a cap on table part count to protect merge performance and metadata size.
number_of_free_entries_in_pool_to_execute_mutation	20	Yes	Uses the upstream default while limiting the value below the service background pool capacity.
number_of_free_entries_in_pool_to_execute_optimize_entire_partition	25	Yes	Uses the upstream default while limiting the value below the service background pool capacity.
number_of_free_entries_in_pool_to_lower_max_size_of_merge	8	Yes	Uses the upstream default while limiting the value below the service background pool capacity.
old_parts_lifetime	60	Yes	Reduces the lifetime of outdated merged parts to lower the number of ZooKeeper or Keeper metadata nodes.

Server settings

These settings are configured at the server level and are managed by Aiven. They are not configurable as session settings. Most settings are derived from your service plan. Some settings, such as vector_similarity_index_cache_size, are exposed through advanced configuration.

Setting	Aiven default	Description
background_pool_size	[8..32, depending on CPU count]	Sizes the background merge and mutation pool based on service CPU capacity. If an explicit service-level override is configured, Aiven preserves it.
cgroups_memory_usage_observer_wait_time	0	Disables the ClickHouse cgroup memory usage observer because Aiven sets explicit memory limits for the managed server process.
cluster_database	default	Sets the database used by cluster-related helpers to the managed default database.
database_atomic_delay_before_drop_table_sec	0	Removes the Atomic database delayed-drop wait to prevent race conditions in refreshable materialized views.
default_database	default	Uses the managed default database unless another database is selected explicitly.
default_replica_name	{replica}	Uses the Aiven-provided replica macro in replicated table paths.
dictionary_user	avnadmin	Runs dictionary queries through the managed admin user.
disable_internal_dns_cache	1	Prevents ClickHouse from keeping stale internal DNS results in a managed environment where node addresses can change.
display_secrets_in_show_and_select	1	Keeps server-level secret display available for privileged managed operations. User-facing profile settings hide secrets by default.
iceberg_catalog_threadpool_pool_size	[3..80, depending on CPU count]	Scales Iceberg catalog worker concurrency with service CPU capacity.
iceberg_metadata_files_cache_size	[~7% of RAM]	Sizes the Iceberg metadata file cache based on the managed memory budget.
index_mark_cache_size	[~7% of RAM]	Sizes the secondary index mark cache based on the managed memory budget.
keep_alive_timeout	10	Limits idle keep-alive connections to 10 seconds so clients cannot hold server resources indefinitely.
mark_cache_size	[~7% of RAM]	Sizes the primary mark cache based on the managed memory budget.
max_concurrent_queries	[147..847, depending on service size]	Sets a server-wide concurrency limit based on the service plan, with extra internal capacity reserved for monitoring, backup, and operator queries.
max_connections	[1000..4000, depending on service size]	Sets the connection limit based on service size. Smaller nodes have lower limits to prevent overload, and larger service plans allow higher limits.
max_database_num_to_throw	400	Enforces a hard limit on the number of databases to protect metadata and reduce operational overhead.
max_database_num_to_warn	200	Warns before the hard database-count limit is reached.
max_format_parsing_thread_pool_size	[3..80, depending on CPU count]	Scales the format parsing thread pool with service CPU capacity.
max_partition_size_to_drop	0	Removes the server-side partition-size guard for drops to avoid common operational issues.
max_prefixes_deserialization_thread_pool_size	[3..80, depending on CPU count]	Scales the prefixes deserialization thread pool with service CPU capacity.
max_server_memory_usage	[65%..70% of host RAM, depending on service size]	Caps ClickHouse memory use below total host RAM so the node retains memory for the operating system, caching, and service management operations.
max_table_size_to_drop	0	Removes the server-side table-size guard for drops to avoid common operational issues.
memory_worker_correct_memory_tracker	1	Lets the ClickHouse memory worker correct memory tracking drift.
prefetch_threadpool_pool_size	[3..80, depending on CPU count]	Scales the prefetch thread pool with service CPU capacity.
prepare_system_log_tables_on_startup	1	Ensures system log tables are ready immediately after startup for managed observability and diagnostics.
reserved_replicated_database_prefixes	["aiven", "endpoint_", "service_"]	Reserves internal database name prefixes for Aiven-managed objects.
series_keeper_path	/clickhouse/series	Sets the Keeper path for generateSerialID counter nodes.
show_addresses_in_stack_traces	0	Avoids exposing raw addresses in stack traces returned to users or logs.
threadpool_local_fs_reader_pool_size	[3..80, depending on CPU count]	Scales local filesystem reader concurrency with service CPU capacity.
threadpool_remote_fs_reader_pool_size	[3..80, depending on CPU count]	Scales remote filesystem reader concurrency with service CPU capacity.
threadpool_writer_pool_size	[3..80, depending on CPU count]	Scales filesystem writer concurrency with service CPU capacity.
uncompressed_cache_size	[~7% of RAM]	Sizes the uncompressed block cache based on the managed memory budget.
user_with_indirect_database_creation	avnadmin	Restricts indirect database creation privileges to the managed admin user.
vector_similarity_index_cache_size	0.07	Sizes the vector similarity index cache as a fraction of total server memory. The default is 0.07, or 7% of RAM. Set to 0 to disable the cache. Maximum value: 0.5. This setting can be changed in advanced configuration and applies to ClickHouse 25.8 or later.