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    PolarDB:Release notes for PolarDB for MySQL 5.6 kernel parameters

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    PolarDB:Release notes for PolarDB for MySQL 5.6 kernel parameters

    Last Updated:Nov 11, 2025

    This topic describes the kernel parameters for PolarDB for MySQL 5.6.

    2025.10.29

    Category

    Parameter

    Level

    Description

    Modified

    default_time_zone

    Global

    Expands the range of valid values for the time zone parameter.

    2025.09.11

    Category

    Parameter

    Level

    Description

    New

    innodb_lru_scan_depth

    Global

    Adjusts the threshold for LRU dirty page flushing. You can now modify this parameter in the console.

    Note

    • The default value and valid range of this parameter are dynamically adjusted based on the specifications of the cluster's compute nodes.

    • Changes to this parameter take effect immediately without a cluster restart.

    2025.06.26

    Category

    Parameter

    Level

    Description

    New

    loose_sql_big_selects

    Session

    Specifies whether to allow the execution of large-scale SELECT queries.

    Valid values:

    • OFF (default): Disables the feature.

    • ON: Enables the feature.

    Note

    • The default value of this parameter depends on the max_join_size parameter value when you connect to the PolarDB cluster. If max_join_size is 18446744073709551615, the default value of this parameter is ON. Otherwise, the default value is OFF.

    • If you manually modify this parameter, the new value persists.

    2025.03.27

    Category

    Parameter

    Level

    Description

    Modified

    innodb_strict_mode

    Global

    Changes the default value from ON to OFF.

    Valid values:

    • OFF (default): Disables the feature.

    • ON: Enables the feature.

    2025.03.20

    Category

    Parameter

    Level

    Description

    New

    loose_innodb_slave_log_apply_worker

    Global

    Specifies the number of Apply threads. You can adjust this parameter in the PolarDB console to improve the replication performance of secondary nodes.

    Valid values: 1 to 64. Default value: 8.

    2025.03.13

    Category

    Parameter

    Level

    Description

    Modified

    loose_innodb_ro_read_no_sync_enable

    Global

    Makes the RO No Sync parameter visible. Enabling this parameter significantly improves the page I/O read performance of read-only nodes.

    Valid values:

    • OFF (default): Disables the feature.

    • ON: Enables the feature.

    Note

    • This parameter is available for the database engine of MySQL 5.6 with a minor version of 5.6.1.0.44 or later.

    • Changes to this parameter take effect immediately without the need to restart the cluster.

    2025.02.12

    Category

    Parameter

    Level

    Description

    Modified

    innodb_online_alter_log_max_size

    Global

    Modifies the maximum size of log files for online index creation.

    Valid values: 65536 to 1099511627775. Default value: 107374182400. Unit: bytes.

    Note

    Changes to this parameter take effect immediately without the need to restart the cluster.

    2025.01.16

    Category

    Parameter

    Level

    Description

    New

    loose_innodb_buffer_pool_instances

    Global

    Sets the number of buffer pool instances.

    Valid values: 1 to 64. Default value: 8.

    2024.09.26

    Category

    Parameter

    Level

    Description

    Modified

    loose_innodb_purge_batch_size

    Global

    Adjusts the default value of the parameter based on the instance type to improve CPU utilization.

    2024.08.22

    Category

    Parameter

    Level

    Description

    Modified

    default_time_zone

    Global/Session

    Sets the time zone for default_time_zone. The range of valid values is expanded. The following formats are supported:

    • PRC

    • Asia/Jakarta

    • Europe/Berlin

    • Asia/Tokyo

    • US/Pacific

    • US/Eastern

    • Europe/London

    • Australia/Sydney

    • Asia/Manila

    • Asia/Bangkok

    • Asia/Seoul

    • America/Los_Angeles

    • America/Vancouver

    • Asia/Singapore

    • Singapore

    • Canada/Atlantic

    • Canada/Central

    • Canada/East-Saskatchewan

    • Canada/Eastern

    • Canada/Mountain

    • Canada/Newfoundland

    • Canada/Pacific

    • Canada/Saskatchewan

    • Canada/Yukon

    2024.08.15

    Category

    Parameter

    Level

    Description

    New

    loose_polar_support_mdl_sync_preemption

    Global/Session

    Specifies whether to enable preemptive DDL.

    • OFF (default): Disables the feature.

    • ON: Enables the feature.

    Note

    This parameter is available for the database engine of MySQL 5.6 with a minor version of 5.6.1.0.43 or later.

    loose_polar_mdl_sync_preempt_after_wait_second

    Global

    Specifies the wait timeout in seconds for a blocked metadata lock (MDL). If the lock is not acquired within this time, the system starts to preempt the thread.

    Valid values: 1 to 31536000. Default value: 10. Unit: seconds.

    Modified

    default_time_zone

    Global/Session

    Sets the time zone for default_time_zone. The range of valid values is expanded. The following formats are supported:

    • PRC

    • Asia/Jakarta

    • Europe/Berlin

    • Asia/Tokyo

    • US/Pacific

    • US/Eastern

    • Europe/London

    • Australia/Sydney

    • Asia/Manila

    • Asia/Bangkok

    • Asia/Seoul

    • America/Los_Angeles

    • Asia/Singapore

    • Singapore

    • Canada/Atlantic

    • Canada/Central

    • Canada/East-Saskatchewan

    • Canada/Eastern

    • Canada/Mountain

    • Canada/Newfoundland

    • Canada/Pacific

    • Canada/Saskatchewan

    • Canada/Yukon

    2024.06.24

    Category

    Parameter

    Level

    Description

    Modified

    innodb_purge_threads

    Global

    The parameter value is adjusted based on the instance type. For example, the value is 4 for a 4-core instance and 8 for an 8-core instance.

    Valid values: 4 to 32.

    innodb_purge_batch_size

    Global

    Changes the default value of the parameter to 1000.

    loose_rds_audit_max_sql_size

    Global

    Increases the maximum length of a single SQL statement in audit logs to 1048576.

    2024.03.29

    Category

    Parameter

    Level

    Description

    New

    init_connect

    Global

    The SQL statement that is automatically executed when a new connection is established. Valid values:

    • '' (default)

    • 'set names utf8mb4'

    • 'set names utf8'

    • 'set default_collation_for_utf8mb4=utf8mb4_general_ci'

    • 'set default_collation_for_utf8mb4=utf8mb4_general_ci;set names utf8mb4'

    • 'set names utf8mb4 collate utf8mb4_general_ci'

    • 'set names utf8mb4 COLLATE utf8mb4_bin'

    • 'set names utf8mb4 collate utf8mb4_unicode_ci'

    2024.03.14

    Category

    Parameter

    Level

    Description

    New

    innodb_undo_log_truncate

    Global

    Specifies whether to enable the undo log recycling feature. Valid values:

    • ON (default): Enables the undo log recycling feature.

    • OFF: Disables the undo log recycling feature.

    2023.12.25

    Category

    Parameter

    Level

    Description

    Modified

    loose_innodb_primary_flush_max_lsn_lag

    Global/Session

    Changes the default value of the loose_innodb_primary_flush_max_lsn_lag parameter to GREATEST(1073741824, {DBNodeClassMemory * 9/100}).

    2023.12.18

    Category

    Parameter

    Level

    Description

    New

    max_connections

    Global/Session

    The maximum number of connections to the database.

    Valid values: 1 to 200000. Default value: 2312.

    max_user_connections

    Global/Session

    The maximum number of connections for a single user.

    Valid values: 10 to 200000. Default value: 1800.

    2023.11.09

    Category

    Parameter

    Level

    Description

    New

    loose_innodb_persist_replica_enable

    Global

    Specifies whether read-only nodes remain available when the read/write node is unavailable. Valid values:

    • OFF (default): Read-only nodes are unavailable when the read/write node is unavailable.

    • ON: Read-only nodes remain available on a best-effort basis when the read/write node is unavailable.

    2023.09.26

    Category

    Parameter

    Level

    Description

    New

    loose_innodb_trx_resume

    Global

    Specifies whether to enable the transaction resumable upload feature. Valid values:

    • OFF (default): Disables the feature.

    • ON: Enables the feature.

    loose_innodb_trx_resume_persist_undo_at_stmt

    Global

    When transaction resumable upload is enabled, specifies whether to synchronously persist undo logs at the statement level. Valid values:

    • ON (default): Undo logs are synchronously persisted at the statement level.

    • OFF: Undo logs are asynchronously persisted.

    2023.08.31

    Category

    Parameter

    Level

    Description

    Modified

    loose_max_statement_time

    Global/Session

    You can configure this parameter on a node.

    2023.08.09

    Category

    Parameter

    Level

    Description

    New

    innodb_ft_server_stopword_table

    Global/Session

    Specifies the stop words for the full-text index.

    2023.06.28

    Category

    Parameter

    Level

    Description

    New

    sort_buffer_size

    Global/Session

    The size of the buffer that is allocated to each session for sorting operations.

    Valid values: 32768 to 4294967295. Default value: 1024 KB. Unit: bytes.

    2023.06.26

    Category

    Parameter

    Level

    Description

    New

    loose_rds_trx_idle_timeout

    Global/Session

    The timeout period for the server to wait for idle transactions.

    Valid values: 0 to 30758400. Default value: 0. Unit: seconds.

    loose_rds_strict_trx_idle_timeout

    Global/Session

    The timeout period for the server to wait for idle transactions in a concurrent state.

    Valid values: 0 to 30758400. Default value: 0. Unit: seconds.

    loose_rds_trx_readonly_idle_timeout

    Global/Session

    The timeout period for the server to wait for read-only idle transactions.

    Valid values: 0 to 30758400. Default value: 0. Unit: seconds.

    loose_rds_trx_changes_idle_timeout

    Global/Session

    The timeout period for the server to wait for non-read-only idle transactions.

    Valid values: 0 to 30758400. Default value: 0. Unit: seconds.

    2023.05.22

    Category

    Parameter

    Level

    Description

    New

    loose_imci_thread_pool_size

    Global

    Specifies the maximum capacity of the thread pool for the In-Memory Column Index (IMCI).

    Valid values: 1 to 2048. Default value: DBNodeClassCPU × 3.

    lower_case_table_names

    Global/Session

    Specifies whether table names are case-sensitive. Valid values:

    • 1 (default): Table names are stored in lowercase and comparisons are not case-sensitive.

    • 0: Table names are stored and compared in a case-sensitive manner.

    • 2: Table names are stored as-is, but comparisons are not case-sensitive.

    2023.03.23

    Category

    Parameter

    Level

    Description

    New

    loose_foreign_key_checks

    Global/Session

    Specifies whether to check foreign key constraints. Valid values:

    • ON (default): Checks foreign key constraints.

    • OFF: Does not check foreign key constraints.

    2022.10.28

    Category

    Parameter

    Level

    Description

    New

    loose_low_limit_check_enabled

    Global

    Specifies whether to use the low limit optimization method. Valid values:

    • ON (default): Uses the low limit optimization method.

    • OFF: Does not use the low limit optimization method.

    loose_prefer_ordering_index

    Global

    Specifies whether to use the ordering index optimization method. Valid values:

    • ON (default): Uses the ordering index optimization method.

    • OFF: Does not use the ordering index optimization method.

    2022.10.24

    Category

    Parameter

    Level

    Description

    Modified

    loose_innodb_logical_read_ahead_threads

    Global

    Adjusts the default number of threads for logical read-ahead.