System variables that are true or false can be enabled at server startup by naming them, or disabled by using a --skip- prefix. For example, to enable or disable the InnoDB adaptive hash index, you can use --innodb-adaptive-hash-index or --skip-innodb-adaptive-hash-index on the command line, or innodb_adaptive_hash_index or skip_innodb_adaptive_hash_index in an option file.

Some variable descriptions refer to “enabling” or “disabling” a variable. These variables can be enabled with the SET statement by setting them to ON or 1, or disabled by setting them to OFF or 0. Boolean variables can be set at startup to the values ON, TRUE, OFF, and FALSE (not case-sensitive), as well as 1 and 0. See Section 6.2.2.4, “Program Option Modifiers”.

System variables that take a numeric value can be specified as --var_name=value on the command line or as var_name=value in option files.

Many system variables can be changed at runtime (see Section 7.1.9.2, “Dynamic System Variables”).

For information about GLOBAL and SESSION variable scope modifiers, refer to the SET statement documentation.

Certain options control the locations and layout of the InnoDB data files. Section 17.8.1, “InnoDB Startup Configuration” explains how to use these options.

Some options, which you might not use initially, help tune InnoDB performance characteristics based on machine capacity and database workload.

For more information on specifying options and system variables, see Section 6.2.2, “Specifying Program Options”.

--innodb-status-file

The --innodb-status-file startup option controls whether InnoDB creates a file named innodb_status.pid in the data directory and writes SHOW ENGINE INNODB STATUS output to it every 15 seconds, approximately.

The innodb_status.pid file is not created by default. To create it, start mysqld with the --innodb-status-file option. InnoDB removes the file when the server is shut down normally. If an abnormal shutdown occurs, the status file may have to be removed manually.

The --innodb-status-file option is intended for temporary use, as SHOW ENGINE INNODB STATUS output generation can affect performance, and the innodb_status.pid file can become quite large over time.

For related information, see Section 17.17.2, “Enabling InnoDB Monitors”.

innodb_adaptive_flushing

Specifies whether to dynamically adjust the rate of flushing dirty pages in the InnoDB buffer pool based on the workload. Adjusting the flush rate dynamically is intended to avoid bursts of I/O activity. This setting is enabled by default. See Section 17.8.3.5, “Configuring Buffer Pool Flushing” for more information. For general I/O tuning advice, see Section 10.5.8, “Optimizing InnoDB Disk I/O”.

innodb_adaptive_flushing_lwm

Defines the low water mark representing percentage of redo log capacity at which adaptive flushing is enabled. For more information, see Section 17.8.3.5, “Configuring Buffer Pool Flushing”.

innodb_adaptive_hash_index

Whether the InnoDB adaptive hash index is enabled or disabled. It may be desirable, depending on your workload, to dynamically enable or disable adaptive hash indexing to improve query performance. Because the adaptive hash index may not be useful for all workloads, conduct benchmarks with it both enabled and disabled, using realistic workloads. See Section 17.5.3, “Adaptive Hash Index” for details.

This variable is enabled by default. You can modify this parameter using the SET GLOBAL statement, without restarting the server. Changing the setting at runtime requires privileges sufficient to set global system variables. See Section 7.1.9.1, “System Variable Privileges”. You can also use --skip-innodb-adaptive-hash-index at server startup to disable it.

Disabling the adaptive hash index empties the hash table immediately. Normal operations can continue while the hash table is emptied, and executing queries that were using the hash table access the index B-trees directly instead. When the adaptive hash index is re-enabled, the hash table is populated again during normal operation.

innodb_adaptive_hash_index_parts

Partitions the adaptive hash index search system. Each index is bound to a specific partition, with each partition protected by a separate latch.

The adaptive hash index search system is partitioned into 8 parts by default. The maximum setting is 512.

For related information, see Section 17.5.3, “Adaptive Hash Index”.

innodb_adaptive_max_sleep_delay

Permits InnoDB to automatically adjust the value of innodb_thread_sleep_delay up or down according to the current workload. Any nonzero value enables automated, dynamic adjustment of the innodb_thread_sleep_delay value, up to the maximum value specified in the innodb_adaptive_max_sleep_delay option. The value represents the number of microseconds. This option can be useful in busy systems, with greater than 16 InnoDB threads. (In practice, it is most valuable for MySQL systems with hundreds or thousands of simultaneous connections.)

For more information, see Section 17.8.4, “Configuring Thread Concurrency for InnoDB”.

innodb_autoextend_increment

The increment size (in megabytes) for extending the size of an auto-extending InnoDB system tablespace file when it becomes full. The default value is 64. For related information, see System Tablespace Data File Configuration, and Resizing the System Tablespace.

The innodb_autoextend_increment setting does not affect file-per-table tablespace files or general tablespace files. These files are auto-extending regardless of the innodb_autoextend_increment setting. The initial extensions are by small amounts, after which extensions occur in increments of 4MB.

innodb_autoinc_lock_mode

The lock mode to use for generating auto-increment values. Permissible values are 0, 1, or 2, for traditional, consecutive, or interleaved, respectively.

The default setting is 2 (interleaved), for compatibility with row-based replication.

For the characteristics of each lock mode, see InnoDB AUTO_INCREMENT Lock Modes.

innodb_background_drop_list_empty

Enabling the innodb_background_drop_list_empty debug option helps avoid test case failures by delaying table creation until the background drop list is empty. For example, if test case A places table t1 on the background drop list, test case B waits until the background drop list is empty before creating table t1.

innodb_buffer_pool_chunk_size

innodb_buffer_pool_chunk_size defines the chunk size for InnoDB buffer pool resizing operations.

To avoid copying all buffer pool pages during resizing operations, the operation is performed in “chunks”. By default, innodb_buffer_pool_chunk_size is 128MB (134217728 bytes). The number of pages contained in a chunk depends on the value of innodb_page_size. innodb_buffer_pool_chunk_size can be increased or decreased in units of 1MB (1048576 bytes).

The following conditions apply when altering the innodb_buffer_pool_chunk_size value:

To avoid potential performance issues, the number of chunks (innodb_buffer_pool_size / innodb_buffer_pool_chunk_size) should not exceed 1000.

The innodb_buffer_pool_size variable is dynamic, which permits resizing the buffer pool while the server is online. However, the buffer pool size must be equal to or a multiple of innodb_buffer_pool_chunk_size * innodb_buffer_pool_instances, and changing either of those variable settings requires restarting the server.

See Section 17.8.3.1, “Configuring InnoDB Buffer Pool Size” for more information.

innodb_buffer_pool_debug

Enabling this option permits multiple buffer pool instances when the buffer pool is less than 1GB in size, ignoring the 1GB minimum buffer pool size constraint imposed on innodb_buffer_pool_instances. The innodb_buffer_pool_debug option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_buffer_pool_dump_at_shutdown

Specifies whether to record the pages cached in the InnoDB buffer pool when the MySQL server is shut down, to shorten the warmup process at the next restart. Typically used in combination with innodb_buffer_pool_load_at_startup. The innodb_buffer_pool_dump_pct option defines the percentage of most recently used buffer pool pages to dump.

Both innodb_buffer_pool_dump_at_shutdown and innodb_buffer_pool_load_at_startup are enabled by default.

For more information, see Section 17.8.3.6, “Saving and Restoring the Buffer Pool State”.

innodb_buffer_pool_dump_now

Immediately makes a record of pages cached in the InnoDB buffer pool. Typically used in combination with innodb_buffer_pool_load_now.

Enabling innodb_buffer_pool_dump_now triggers the recording action but does not alter the variable setting, which always remains OFF or 0. To view buffer pool dump status after triggering a dump, query the Innodb_buffer_pool_dump_status variable.

Enabling innodb_buffer_pool_dump_now triggers the dump action but does not alter the variable setting, which always remains OFF or 0. To view buffer pool dump status after triggering a dump, query the Innodb_buffer_pool_dump_status variable.

For more information, see Section 17.8.3.6, “Saving and Restoring the Buffer Pool State”.

innodb_buffer_pool_dump_pct

Specifies the percentage of the most recently used pages for each buffer pool to read out and dump. The range is 1 to 100. The default value is 25. For example, if there are 4 buffer pools with 100 pages each, and innodb_buffer_pool_dump_pct is set to 25, the 25 most recently used pages from each buffer pool are dumped.

innodb_buffer_pool_filename

Specifies the name of the file that holds the list of tablespace IDs and page IDs produced by innodb_buffer_pool_dump_at_shutdown or innodb_buffer_pool_dump_now. Tablespace IDs and page IDs are saved in the following format: space, page_id. By default, the file is named ib_buffer_pool and is located in the InnoDB data directory. A non-default location must be specified relative to the data directory.

A file name can be specified at runtime, using a SET statement:

SET GLOBAL innodb_buffer_pool_filename='file_name';

You can also specify a file name at startup, in a startup string or MySQL configuration file. When specifying a file name at startup, the file must exist or InnoDB returns a startup error indicating that there is no such file or directory.

For more information, see Section 17.8.3.6, “Saving and Restoring the Buffer Pool State”.

innodb_buffer_pool_in_core_file

Disabling the innodb_buffer_pool_in_core_file variable reduces the size of core files by excluding InnoDB buffer pool pages. To use this variable, the core_file variable must be enabled and the operating system must support the MADV_DONTDUMP non-POSIX extension to madvise(), which is supported in Linux 3.4 and later. For more information, see Section 17.8.3.7, “Excluding Buffer Pool Pages from Core Files”.

innodb_buffer_pool_instances

The number of regions that the InnoDB buffer pool is divided into. For systems with buffer pools in the multi-gigabyte range, dividing the buffer pool into separate instances can improve concurrency, by reducing contention as different threads read and write to cached pages. Each page that is stored in or read from the buffer pool is assigned to one of the buffer pool instances randomly, using a hashing function. Each buffer pool manages its own free lists, flush lists, LRUs, and all other data structures connected to a buffer pool, and is protected by its own buffer pool mutex.

This option only takes effect when setting innodb_buffer_pool_size to 1GB or more. The total buffer pool size is divided among all the buffer pools. For best efficiency, specify a combination of innodb_buffer_pool_instances and innodb_buffer_pool_size so that each buffer pool instance is at least 1GB.

The default value on 32-bit Windows systems depends on the value of innodb_buffer_pool_size, as described below:

On all other platforms, the default value is 8 when innodb_buffer_pool_size is greater than or equal to 1GB. Otherwise, the default is 1.

For related information, see Section 17.8.3.1, “Configuring InnoDB Buffer Pool Size”.

innodb_buffer_pool_load_abort

Interrupts the process of restoring InnoDB buffer pool contents triggered by innodb_buffer_pool_load_at_startup or innodb_buffer_pool_load_now.

Enabling innodb_buffer_pool_load_abort triggers the abort action but does not alter the variable setting, which always remains OFF or 0. To view buffer pool load status after triggering an abort action, query the Innodb_buffer_pool_load_status variable.

For more information, see Section 17.8.3.6, “Saving and Restoring the Buffer Pool State”.

innodb_buffer_pool_load_at_startup

Specifies that, on MySQL server startup, the InnoDB buffer pool is automatically warmed up by loading the same pages it held at an earlier time. Typically used in combination with innodb_buffer_pool_dump_at_shutdown.

Both innodb_buffer_pool_dump_at_shutdown and innodb_buffer_pool_load_at_startup are enabled by default.

For more information, see Section 17.8.3.6, “Saving and Restoring the Buffer Pool State”.

innodb_buffer_pool_load_now

Immediately warms up the InnoDB buffer pool by loading data pages without waiting for a server restart. Can be useful to bring cache memory back to a known state during benchmarking or to ready the MySQL server to resume its normal workload after running queries for reports or maintenance.

Enabling innodb_buffer_pool_load_now triggers the load action but does not alter the variable setting, which always remains OFF or 0. To view buffer pool load progress after triggering a load, query the Innodb_buffer_pool_load_status variable.

For more information, see Section 17.8.3.6, “Saving and Restoring the Buffer Pool State”.

innodb_buffer_pool_size

The size in bytes of the buffer pool, the memory area where InnoDB caches table and index data. The default value is 134217728 bytes (128MB). The maximum value depends on the CPU architecture; the maximum is 4294967295 (2³²-1) on 32-bit systems and 18446744073709551615 (2⁶⁴-1) on 64-bit systems. On 32-bit systems, the CPU architecture and operating system may impose a lower practical maximum size than the stated maximum. When the size of the buffer pool is greater than 1GB, setting innodb_buffer_pool_instances to a value greater than 1 can improve the scalability on a busy server.

A larger buffer pool requires less disk I/O to access the same table data more than once. On a dedicated database server, you might set the buffer pool size to 80% of the machine's physical memory size. Be aware of the following potential issues when configuring buffer pool size, and be prepared to scale back the size of the buffer pool if necessary.

When you increase or decrease buffer pool size, the operation is performed in chunks. Chunk size is defined by the innodb_buffer_pool_chunk_size variable, which has a default of 128 MB.

Buffer pool size must always be equal to or a multiple of innodb_buffer_pool_chunk_size * innodb_buffer_pool_instances. If you alter the buffer pool size to a value that is not equal to or a multiple of innodb_buffer_pool_chunk_size * innodb_buffer_pool_instances, buffer pool size is automatically adjusted to a value that is equal to or a multiple of innodb_buffer_pool_chunk_size * innodb_buffer_pool_instances.

innodb_buffer_pool_size can be set dynamically, which allows you to resize the buffer pool without restarting the server. The Innodb_buffer_pool_resize_status status variable reports the status of online buffer pool resizing operations. See Section 17.8.3.1, “Configuring InnoDB Buffer Pool Size” for more information.

If innodb_dedicated_server is enabled, the innodb_buffer_pool_size value is automatically configured if it is not explicitly defined. For more information, see Section 17.8.12, “Enabling Automatic Configuration for a Dedicated MySQL Server”.

innodb_change_buffer_max_size

Maximum size for the InnoDB change buffer, as a percentage of the total size of the buffer pool. You might increase this value for a MySQL server with heavy insert, update, and delete activity, or decrease it for a MySQL server with unchanging data used for reporting. For more information, see Section 17.5.2, “Change Buffer”. For general I/O tuning advice, see Section 10.5.8, “Optimizing InnoDB Disk I/O”.

innodb_change_buffering

Whether InnoDB performs change buffering, an optimization that delays write operations to secondary indexes so that the I/O operations can be performed sequentially. Permitted values are described in the following table. Values may also be specified numerically.

For more information, see Section 17.5.2, “Change Buffer”. For general I/O tuning advice, see Section 10.5.8, “Optimizing InnoDB Disk I/O”.

innodb_change_buffering_debug

Sets a debug flag for InnoDB change buffering. A value of 1 forces all changes to the change buffer. A value of 2 causes an unexpected exit at merge. A default value of 0 indicates that the change buffering debug flag is not set. This option is only available when debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_checkpoint_disabled

This is a debug option that is only intended for expert debugging use. It disables checkpoints so that a deliberate server exit always initiates InnoDB recovery. It should only be enabled for a short interval, typically before running DML operations that write redo log entries that would require recovery following a server exit. This option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_checksum_algorithm

Specifies how to generate and verify the checksum stored in the disk blocks of InnoDB tablespaces. The default value for innodb_checksum_algorithm is crc32.

The value innodb is backward-compatible with earlier versions of MySQL. The value crc32 uses an algorithm that is faster to compute the checksum for every modified block, and to check the checksums for each disk read. It scans blocks 64 bits at a time, which is faster than the innodb checksum algorithm, which scans blocks 8 bits at a time. The value none writes a constant value in the checksum field rather than computing a value based on the block data. The blocks in a tablespace can use a mix of old, new, and no checksum values, being updated gradually as the data is modified; once blocks in a tablespace are modified to use the crc32 algorithm, the associated tables cannot be read by earlier versions of MySQL.

The strict form of a checksum algorithm reports an error if it encounters a valid but non-matching checksum value in a tablespace. It is recommended that you only use strict settings in a new instance, to set up tablespaces for the first time. Strict settings are somewhat faster, because they do not need to compute all checksum values during disk reads.

The following table shows the difference between the none, innodb, and crc32 option values, and their strict counterparts. none, innodb, and crc32 write the specified type of checksum value into each data block, but for compatibility accept other checksum values when verifying a block during a read operation. Strict settings also accept valid checksum values but print an error message when a valid non-matching checksum value is encountered. Using the strict form can make verification faster if all InnoDB data files in an instance are created under an identical innodb_checksum_algorithm value.

innodb_cmp_per_index_enabled

Enables per-index compression-related statistics in the Information Schema INNODB_CMP_PER_INDEX table. Because these statistics can be expensive to gather, only enable this option on development, test, or replica instances during performance tuning related to InnoDB compressed tables.

For more information, see Section 28.4.8, “The INFORMATION_SCHEMA INNODB_CMP_PER_INDEX and INNODB_CMP_PER_INDEX_RESET Tables”, and Section 17.9.1.4, “Monitoring InnoDB Table Compression at Runtime”.

innodb_commit_concurrency

The number of threads that can commit at the same time. A value of 0 (the default) permits any number of transactions to commit simultaneously.

The value of innodb_commit_concurrency cannot be changed at runtime from zero to nonzero or vice versa. The value can be changed from one nonzero value to another.

innodb_compress_debug

Compresses all tables using a specified compression algorithm without having to define a COMPRESSION attribute for each table. This option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

For related information, see Section 17.9.2, “InnoDB Page Compression”.

innodb_compression_failure_threshold_pct

Defines the compression failure rate threshold for a table, as a percentage, at which point MySQL begins adding padding within compressed pages to avoid expensive compression failures. When this threshold is passed, MySQL begins to leave additional free space within each new compressed page, dynamically adjusting the amount of free space up to the percentage of page size specified by innodb_compression_pad_pct_max. A value of zero disables the mechanism that monitors compression efficiency and dynamically adjusts the padding amount.

For more information, see Section 17.9.1.6, “Compression for OLTP Workloads”.

innodb_compression_level

Specifies the level of zlib compression to use for InnoDB compressed tables and indexes. A higher value lets you fit more data onto a storage device, at the expense of more CPU overhead during compression. A lower value lets you reduce CPU overhead when storage space is not critical, or you expect the data is not especially compressible.

For more information, see Section 17.9.1.6, “Compression for OLTP Workloads”.

innodb_compression_pad_pct_max

Specifies the maximum percentage that can be reserved as free space within each compressed page, allowing room to reorganize the data and modification log within the page when a compressed table or index is updated and the data might be recompressed. Only applies when innodb_compression_failure_threshold_pct is set to a nonzero value, and the rate of compression failures passes the cutoff point.

For more information, see Section 17.9.1.6, “Compression for OLTP Workloads”.

innodb_concurrency_tickets

Determines the number of threads that can enter InnoDB concurrently. A thread is placed in a queue when it tries to enter InnoDB if the number of threads has already reached the concurrency limit. When a thread is permitted to enter InnoDB, it is given a number of “ tickets” equal to the value of innodb_concurrency_tickets, and the thread can enter and leave InnoDB freely until it has used up its tickets. After that point, the thread again becomes subject to the concurrency check (and possible queuing) the next time it tries to enter InnoDB. The default value is 5000.

With a small innodb_concurrency_tickets value, small transactions that only need to process a few rows compete fairly with larger transactions that process many rows. The disadvantage of a small innodb_concurrency_tickets value is that large transactions must loop through the queue many times before they can complete, which extends the amount of time required to complete their task.

With a large innodb_concurrency_tickets value, large transactions spend less time waiting for a position at the end of the queue (controlled by innodb_thread_concurrency) and more time retrieving rows. Large transactions also require fewer trips through the queue to complete their task. The disadvantage of a large innodb_concurrency_tickets value is that too many large transactions running at the same time can starve smaller transactions by making them wait a longer time before executing.

With a nonzero innodb_thread_concurrency value, you may need to adjust the innodb_concurrency_tickets value up or down to find the optimal balance between larger and smaller transactions. The SHOW ENGINE INNODB STATUS report shows the number of tickets remaining for an executing transaction in its current pass through the queue. This data may also be obtained from the TRX_CONCURRENCY_TICKETS column of the Information Schema INNODB_TRX table.

For more information, see Section 17.8.4, “Configuring Thread Concurrency for InnoDB”.

innodb_data_file_path

Defines the name, size, and attributes of InnoDB system tablespace data files. If you do not specify a value for innodb_data_file_path, the default behavior is to create a single auto-extending data file, slightly larger than 12MB, named ibdata1.

The full syntax for a data file specification includes the file name, file size, autoextend attribute, and max attribute:

file_name:file_size[:autoextend[:max:max_file_size]]

File sizes are specified in kilobytes, megabytes, or gigabytes by appending K, M or G to the size value. If specifying the data file size in kilobytes, do so in multiples of 1024. Otherwise, KB values are rounded to nearest megabyte (MB) boundary. The sum of file sizes must be, at a minimum, slightly larger than 12MB.

For additional configuration information, see System Tablespace Data File Configuration. For resizing instructions, see Resizing the System Tablespace.

innodb_data_home_dir

The common part of the directory path for InnoDB system tablespace data files. The default value is the MySQL data directory. The setting is concatenated with the innodb_data_file_path setting, unless that setting is defined with an absolute path.

A trailing slash is required when specifying a value for innodb_data_home_dir. For example:

[mysqld]
innodb_data_home_dir = /path/to/myibdata/

This setting does not affect the location of file-per-table tablespaces.

For related information, see Section 17.8.1, “InnoDB Startup Configuration”.

innodb_ddl_buffer_size

Defines the maximum buffer size for DDL operations. The default setting is 1048576 bytes (approximately 1 MB). Applies to online DDL operations that create or rebuild secondary indexes. See Section 17.12.4, “Online DDL Memory Management”. The maximum buffer size per DDL thread is the maximum buffer size divided by the number of DDL threads (innodb_ddl_buffer_size/innodb_ddl_threads).

innodb_ddl_log_crash_reset_debug

Enable this debug option to reset DDL log crash injection counters to 1. This option is only available when debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_ddl_threads

Defines the maximum number of parallel threads for the sort and build phases of index creation. Applies to online DDL operations that create or rebuild secondary indexes. For related information, see Section 17.12.5, “Configuring Parallel Threads for Online DDL Operations”, and Section 17.12.4, “Online DDL Memory Management”.

innodb_deadlock_detect

This option is used to disable deadlock detection. On high concurrency systems, deadlock detection can cause a slowdown when numerous threads wait for the same lock. At times, it may be more efficient to disable deadlock detection and rely on the innodb_lock_wait_timeout setting for transaction rollback when a deadlock occurs.

For related information, see Section 17.7.5.2, “Deadlock Detection”.

innodb_dedicated_server

When innodb_dedicated_server is enabled, InnoDB automatically configures the following variables:

Only consider enabling innodb_dedicated_server if the MySQL instance resides on a dedicated server where it can use all available system resources. Enabling innodb_dedicated_server is not recommended if the MySQL instance shares system resources with other applications.

For more information, see Section 17.8.12, “Enabling Automatic Configuration for a Dedicated MySQL Server”.

innodb_default_row_format

The innodb_default_row_format option defines the default row format for InnoDB tables and user-created temporary tables. The default setting is DYNAMIC. Other permitted values are COMPACT and REDUNDANT. The COMPRESSED row format, which is not supported for use in the system tablespace, cannot be defined as the default.

Newly created tables use the row format defined by innodb_default_row_format when a ROW_FORMAT option is not specified explicitly or when ROW_FORMAT=DEFAULT is used.

When a ROW_FORMAT option is not specified explicitly or when ROW_FORMAT=DEFAULT is used, any operation that rebuilds a table also silently changes the row format of the table to the format defined by innodb_default_row_format. For more information, see Defining the Row Format of a Table.

Internal InnoDB temporary tables created by the server to process queries use the DYNAMIC row format, regardless of the innodb_default_row_format setting.

innodb_directories

Defines directories to scan at startup for tablespace files. This option is used when moving or restoring tablespace files to a new location while the server is offline. It is also used to specify directories of tablespace files created using an absolute path or that reside outside of the data directory.

Tablespace discovery during crash recovery relies on the innodb_directories setting to identify tablespaces referenced in the redo logs. For more information, see Tablespace Discovery During Crash Recovery.

The default value is NULL, but directories defined by innodb_data_home_dir, innodb_undo_directory, and datadir are always appended to the innodb_directories argument value when InnoDB builds a list of directories to scan at startup. These directories are appended regardless of whether an innodb_directories setting is specified explicitly.

innodb_directories may be specified as an option in a startup command or in a MySQL option file. Quotes surround the argument value because otherwise some command interpreters interpret semicolon (;) as a special character. (For example, Unix shells treat it as a command terminator.)

Startup command:

mysqld --innodb-directories="directory_path_1;directory_path_2"

MySQL option file:

[mysqld]
innodb_directories="directory_path_1;directory_path_2"

Wildcard expressions cannot be used to specify directories.

The innodb_directories scan also traverses the subdirectories of specified directories. Duplicate directories and subdirectories are discarded from the list of directories to be scanned.

For more information, see Section 17.6.3.6, “Moving Tablespace Files While the Server is Offline”.

innodb_disable_sort_file_cache

Disables the operating system file system cache for merge-sort temporary files. The effect is to open such files with the equivalent of O_DIRECT.

innodb_doublewrite

The innodb_doublewrite variable controls doublewrite buffering. Doublewrite buffering is enabled by default in most cases.

You can set innodb_doublewrite to ON or OFF when starting the server to enable or disable doublewrite buffering, respectively. DETECT_AND_RECOVER is the same as ON. With this setting, except that the doublewrite buffer is fully enabled, with database page content written to the doublewrite buffer where it is accessed during recovery to fix incomplete page writes. With DETECT_ONLY, only metadata is written to the doublewrite buffer. Database page content is not written to the doublewrite buffer, and recovery does not use the doublewrite buffer to fix incomplete page writes. This lightweight setting is intended for detecting incomplete page writes only.

MySQL 8.3 supports dynamic changes to the innodb_doublewrite setting that enables the doublewrite buffer, between ON, DETECT_AND_RECOVER, and DETECT_ONLY. MySQL does not support dynamic changes between a setting that enables the doublewrite buffer and OFF or vice versa.

If the doublewrite buffer is located on a Fusion-io device that supports atomic writes, the doublewrite buffer is automatically disabled and data file writes are performed using Fusion-io atomic writes instead. However, be aware that the innodb_doublewrite setting is global. When the doublewrite buffer is disabled, it is disabled for all data files including those that do not reside on Fusion-io hardware. This feature is only supported on Fusion-io hardware and is only enabled for Fusion-io NVMFS on Linux. To take full advantage of this feature, an innodb_flush_method setting of O_DIRECT is recommended.

For related information, see Section 17.6.4, “Doublewrite Buffer”.

innodb_doublewrite_batch_size

Defines the number of doublewrite pages to write in a batch.

For more information, see Section 17.6.4, “Doublewrite Buffer”.

innodb_doublewrite_dir

Defines the directory for doublewrite files. If no directory is specified, doublewrite files are created in the innodb_data_home_dir directory, which defaults to the data directory if unspecified.

For more information, see Section 17.6.4, “Doublewrite Buffer”.

innodb_doublewrite_files

Defines the number of doublewrite files. By default, two doublewrite files are created for each buffer pool instance.

At a minimum, there are two doublewrite files. The maximum number of doublewrite files is two times the number of buffer pool instances. (The number of buffer pool instances is controlled by the innodb_buffer_pool_instances variable.)

For more information, see Section 17.6.4, “Doublewrite Buffer”.

innodb_doublewrite_pages

Defines the maximum number of doublewrite pages per thread for a batch write. If no value is specified, innodb_doublewrite_pages is set to the innodb_write_io_threads value.

For more information, see Section 17.6.4, “Doublewrite Buffer”.

innodb_extend_and_initialize

Controls how space is allocated to file-per-table and general tablespaces on Linux systems.

When enabled, InnoDB writes NULLs to newly allocated pages. When disabled, space is allocated using posix_fallocate() calls, which reserve space without physically writing NULLs.

For more information, see Section 17.6.3.8, “Optimizing Tablespace Space Allocation on Linux”.

innodb_fast_shutdown

The InnoDB shutdown mode. If the value is 0, InnoDB does a slow shutdown, a full purge and a change buffer merge before shutting down. If the value is 1 (the default), InnoDB skips these operations at shutdown, a process known as a fast shutdown. If the value is 2, InnoDB flushes its logs and shuts down cold, as if MySQL had crashed; no committed transactions are lost, but the crash recovery operation makes the next startup take longer.

The slow shutdown can take minutes, or even hours in extreme cases where substantial amounts of data are still buffered. Use the slow shutdown technique before upgrading or downgrading between MySQL major releases, so that all data files are fully prepared in case the upgrade process updates the file format.

Use innodb_fast_shutdown=2 in emergency or troubleshooting situations, to get the absolute fastest shutdown if data is at risk of corruption.

innodb_fil_make_page_dirty_debug

By default, setting innodb_fil_make_page_dirty_debug to the ID of a tablespace immediately dirties the first page of the tablespace. If innodb_saved_page_number_debug is set to a non-default value, setting innodb_fil_make_page_dirty_debug dirties the specified page. The innodb_fil_make_page_dirty_debug option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_file_per_table

When innodb_file_per_table is enabled, tables are created in file-per-table tablespaces by default. When disabled, tables are created in the system tablespace by default. For information about file-per-table tablespaces, see Section 17.6.3.2, “File-Per-Table Tablespaces”. For information about the InnoDB system tablespace, see Section 17.6.3.1, “The System Tablespace”.

The innodb_file_per_table variable can be configured at runtime using a SET GLOBAL statement, specified on the command line at startup, or specified in an option file. Configuration at runtime requires privileges sufficient to set global system variables (see Section 7.1.9.1, “System Variable Privileges”) and immediately affects the operation of all connections.

When a table that resides in a file-per-table tablespace is truncated or dropped, the freed space is returned to the operating system. Truncating or dropping a table that resides in the system tablespace only frees space in the system tablespace. Freed space in the system tablespace can be used again for InnoDB data but is not returned to the operating system, as system tablespace data files never shrink.

The innodb_file_per-table setting does not affect the creation of temporary tables; temporary tables are created in session temporary tablespaces. See Section 17.6.3.5, “Temporary Tablespaces”.

innodb_fill_factor

InnoDB performs a bulk load when creating or rebuilding indexes. This method of index creation is known as a “sorted index build”.

innodb_fill_factor defines the percentage of space on each B-tree page that is filled during a sorted index build, with the remaining space reserved for future index growth. For example, setting innodb_fill_factor to 80 reserves 20 percent of the space on each B-tree page for future index growth. Actual percentages may vary. The innodb_fill_factor setting is interpreted as a hint rather than a hard limit.

An innodb_fill_factor setting of 100 leaves 1/16 of the space in clustered index pages free for future index growth.

innodb_fill_factor applies to both B-tree leaf and non-leaf pages. It does not apply to external pages used for TEXT or BLOB entries.

For more information, see Section 17.6.2.3, “Sorted Index Builds”.

innodb_flush_log_at_timeout

Write and flush the logs every N seconds. innodb_flush_log_at_timeout allows the timeout period between flushes to be increased in order to reduce flushing and avoid impacting performance of binary log group commit. The default setting for innodb_flush_log_at_timeout is once per second.

innodb_flush_log_at_trx_commit

Controls the balance between strict ACID compliance for commit operations and higher performance that is possible when commit-related I/O operations are rearranged and done in batches. You can achieve better performance by changing the default value but then you can lose transactions in a crash.

For durability and consistency in a replication setup that uses InnoDB with transactions:

For information on the combination of settings on a replica that is most resilient to unexpected halts, see Section 19.4.2, “Handling an Unexpected Halt of a Replica”.

innodb_flush_method

Defines the method used to flush data to InnoDB data files and log files, which can affect I/O throughput.

On Unix-like systems, the default value is fsync. On Windows, the default value is unbuffered.

The innodb_flush_method options for Unix-like systems include:

On platforms that support fdatasync() system calls, the innodb_use_fdatasync variable permits innodb_flush_method options that use fsync() to use fdatasync() instead. An fdatasync() system call does not flush changes to file metadata unless required for subsequent data retrieval, providing a potential performance benefit.

The innodb_flush_method options for Windows systems include:

How each setting affects performance depends on hardware configuration and workload. Benchmark your particular configuration to decide which setting to use, or whether to keep the default setting. Examine the Innodb_data_fsyncs status variable to see the overall number of fsync() calls (or fdatasync() calls if innodb_use_fdatasync is enabled) for each setting. The mix of read and write operations in your workload can affect how a setting performs. For example, on a system with a hardware RAID controller and battery-backed write cache, O_DIRECT can help to avoid double buffering between the InnoDB buffer pool and the operating system file system cache. On some systems where InnoDB data and log files are located on a SAN, the default value or O_DSYNC might be faster for a read-heavy workload with mostly SELECT statements. Always test this parameter with hardware and workload that reflect your production environment. For general I/O tuning advice, see Section 10.5.8, “Optimizing InnoDB Disk I/O”.

If innodb_dedicated_server is enabled, the innodb_flush_method value is automatically configured if it is not explicitly defined. For more information, see Section 17.8.12, “Enabling Automatic Configuration for a Dedicated MySQL Server”.

innodb_flush_neighbors

Specifies whether flushing a page from the InnoDB buffer pool also flushes other dirty pages in the same extent.

When the table data is stored on a traditional HDD storage device, flushing such neighbor pages in one operation reduces I/O overhead (primarily for disk seek operations) compared to flushing individual pages at different times. For table data stored on SSD, seek time is not a significant factor and you can set this option to 0 to spread out write operations. For related information, see Section 17.8.3.5, “Configuring Buffer Pool Flushing”.

innodb_flush_sync

The innodb_flush_sync variable, which is enabled by default, causes the innodb_io_capacity setting to be ignored during bursts of I/O activity that occur at checkpoints. To adhere to the I/O rate defined by the innodb_io_capacity setting, disable innodb_flush_sync.

For information about configuring the innodb_flush_sync variable, see Section 17.8.7, “Configuring InnoDB I/O Capacity”.

innodb_flushing_avg_loops

Number of iterations for which InnoDB keeps the previously calculated snapshot of the flushing state, controlling how quickly adaptive flushing responds to changing workloads. Increasing the value makes the rate of flush operations change smoothly and gradually as the workload changes. Decreasing the value makes adaptive flushing adjust quickly to workload changes, which can cause spikes in flushing activity if the workload increases and decreases suddenly.

For related information, see Section 17.8.3.5, “Configuring Buffer Pool Flushing”.

innodb_force_load_corrupted

Permits InnoDB to load tables at startup that are marked as corrupted. Use only during troubleshooting, to recover data that is otherwise inaccessible. When troubleshooting is complete, disable this setting and restart the server.

innodb_force_recovery

The crash recovery mode, typically only changed in serious troubleshooting situations. Possible values are from 0 to 6. For the meanings of these values and important information about innodb_force_recovery, see Section 17.20.3, “Forcing InnoDB Recovery”.

innodb_fsync_threshold

By default, when InnoDB creates a new data file, such as a new log file or tablespace file, the file is fully written to the operating system cache before it is flushed to disk, which can cause a large amount of disk write activity to occur at once. To force smaller, periodic flushes of data from the operating system cache, you can use the innodb_fsync_threshold variable to define a threshold value, in bytes. When the byte threshold is reached, the contents of the operating system cache are flushed to disk. The default value of 0 forces the default behavior, which is to flush data to disk only after a file is fully written to the cache.

Specifying a threshold to force smaller, periodic flushes may be beneficial in cases where multiple MySQL instances use the same storage devices. For example, creating a new MySQL instance and its associated data files could cause large surges of disk write activity, impeding the performance of other MySQL instances that use the same storage devices. Configuring a threshold helps avoid such surges in write activity.

innodb_ft_aux_table

Specifies the qualified name of an InnoDB table containing a FULLTEXT index. This variable is intended for diagnostic purposes and can only be set at runtime. For example:

SET GLOBAL innodb_ft_aux_table = 'test/t1';

After you set this variable to a name in the format db_name/table_name, the INFORMATION_SCHEMA tables INNODB_FT_INDEX_TABLE, INNODB_FT_INDEX_CACHE, INNODB_FT_CONFIG, INNODB_FT_DELETED, and INNODB_FT_BEING_DELETED show information about the search index for the specified table.

For more information, see Section 17.15.4, “InnoDB INFORMATION_SCHEMA FULLTEXT Index Tables”.

innodb_ft_cache_size

The memory allocated, in bytes, for the InnoDB FULLTEXT search index cache, which holds a parsed document in memory while creating an InnoDB FULLTEXT index. Index inserts and updates are only committed to disk when the innodb_ft_cache_size size limit is reached. innodb_ft_cache_size defines the cache size on a per table basis. To set a global limit for all tables, see innodb_ft_total_cache_size.

For more information, see InnoDB Full-Text Index Cache.

innodb_ft_enable_diag_print

Whether to enable additional full-text search (FTS) diagnostic output. This option is primarily intended for advanced FTS debugging and is not of interest to most users. Output is printed to the error log and includes information such as:

innodb_ft_enable_stopword

Specifies that a set of stopwords is associated with an InnoDB FULLTEXT index at the time the index is created. If the innodb_ft_user_stopword_table option is set, the stopwords are taken from that table. Else, if the innodb_ft_server_stopword_table option is set, the stopwords are taken from that table. Otherwise, a built-in set of default stopwords is used.

For more information, see Section 14.9.4, “Full-Text Stopwords”.

innodb_ft_max_token_size

Maximum character length of words that are stored in an InnoDB FULLTEXT index. Setting a limit on this value reduces the size of the index, thus speeding up queries, by omitting long keywords or arbitrary collections of letters that are not real words and are not likely to be search terms.

For more information, see Section 14.9.6, “Fine-Tuning MySQL Full-Text Search”.

innodb_ft_min_token_size

Minimum length of words that are stored in an InnoDB FULLTEXT index. Increasing this value reduces the size of the index, thus speeding up queries, by omitting common words that are unlikely to be significant in a search context, such as the English words “a” and “to”. For content using a CJK (Chinese, Japanese, Korean) character set, specify a value of 1.

For more information, see Section 14.9.6, “Fine-Tuning MySQL Full-Text Search”.

innodb_ft_num_word_optimize

Number of words to process during each OPTIMIZE TABLE operation on an InnoDB FULLTEXT index. Because a bulk insert or update operation to a table containing a full-text search index could require substantial index maintenance to incorporate all changes, you might do a series of OPTIMIZE TABLE statements, each picking up where the last left off.

For more information, see Section 14.9.6, “Fine-Tuning MySQL Full-Text Search”.

innodb_ft_result_cache_limit

The InnoDB full-text search query result cache limit (defined in bytes) per full-text search query or per thread. Intermediate and final InnoDB full-text search query results are handled in memory. Use innodb_ft_result_cache_limit to place a size limit on the full-text search query result cache to avoid excessive memory consumption in case of very large InnoDB full-text search query results (millions or hundreds of millions of rows, for example). Memory is allocated as required when a full-text search query is processed. If the result cache size limit is reached, an error is returned indicating that the query exceeds the maximum allowed memory.

The maximum value of innodb_ft_result_cache_limit for all platform types and bit sizes is 2**32-1.

innodb_ft_server_stopword_table

This option is used to specify your own InnoDB FULLTEXT index stopword list for all InnoDB tables. To configure your own stopword list for a specific InnoDB table, use innodb_ft_user_stopword_table.

Set innodb_ft_server_stopword_table to the name of the table containing a list of stopwords, in the format db_name/table_name.

The stopword table must exist before you configure innodb_ft_server_stopword_table. innodb_ft_enable_stopword must be enabled and innodb_ft_server_stopword_table option must be configured before you create the FULLTEXT index.

The stopword table must be an InnoDB table, containing a single VARCHAR column named value.

For more information, see Section 14.9.4, “Full-Text Stopwords”.

innodb_ft_sort_pll_degree

Number of threads used in parallel to index and tokenize text in an InnoDB FULLTEXT index when building a search index.

For related information, see Section 17.6.2.4, “InnoDB Full-Text Indexes”, and innodb_sort_buffer_size.

innodb_ft_total_cache_size

The total memory allocated, in bytes, for the InnoDB full-text search index cache for all tables. Creating numerous tables, each with a FULLTEXT search index, could consume a significant portion of available memory. innodb_ft_total_cache_size defines a global memory limit for all full-text search indexes to help avoid excessive memory consumption. If the global limit is reached by an index operation, a forced sync is triggered.

For more information, see InnoDB Full-Text Index Cache.

innodb_ft_user_stopword_table

This option is used to specify your own InnoDB FULLTEXT index stopword list on a specific table. To configure your own stopword list for all InnoDB tables, use innodb_ft_server_stopword_table.

Set innodb_ft_user_stopword_table to the name of the table containing a list of stopwords, in the format db_name/table_name.

The stopword table must exist before you configure innodb_ft_user_stopword_table. innodb_ft_enable_stopword must be enabled and innodb_ft_user_stopword_table must be configured before you create the FULLTEXT index.

The stopword table must be an InnoDB table, containing a single VARCHAR column named value.

For more information, see Section 14.9.4, “Full-Text Stopwords”.

innodb_idle_flush_pct

Limits page flushing when InnoDB is idle. The innodb_idle_flush_pct value is a percentage of the innodb_io_capacity setting, which defines the number of I/O operations per second available to InnoDB. For more information, see Limiting Buffer Flushing During Idle Periods.

innodb_io_capacity

The innodb_io_capacity variable defines the number of I/O operations per second (IOPS) available to InnoDB background tasks, such as flushing pages from the buffer pool and merging data from the change buffer.

For information about configuring the innodb_io_capacity variable, see Section 17.8.7, “Configuring InnoDB I/O Capacity”.

innodb_io_capacity_max

If flushing activity falls behind, InnoDB can flush more aggressively, at a higher rate of I/O operations per second (IOPS) than defined by the innodb_io_capacity variable. The innodb_io_capacity_max variable defines a maximum number of IOPS performed by InnoDB background tasks in such situations.

For information about configuring the innodb_io_capacity_max variable, see Section 17.8.7, “Configuring InnoDB I/O Capacity”.

innodb_limit_optimistic_insert_debug

Limits the number of records per B-tree page. A default value of 0 means that no limit is imposed. This option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_lock_wait_timeout

The length of time in seconds an InnoDB transaction waits for a row lock before giving up. The default value is 50 seconds. A transaction that tries to access a row that is locked by another InnoDB transaction waits at most this many seconds for write access to the row before issuing the following error:

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

When a lock wait timeout occurs, the current statement is rolled back (not the entire transaction). To have the entire transaction roll back, start the server with the --innodb-rollback-on-timeout option. See also Section 17.20.5, “InnoDB Error Handling”.

You might decrease this value for highly interactive applications or OLTP systems, to display user feedback quickly or put the update into a queue for processing later. You might increase this value for long-running back-end operations, such as a transform step in a data warehouse that waits for other large insert or update operations to finish.

innodb_lock_wait_timeout applies to InnoDB row locks. A MySQL table lock does not happen inside InnoDB and this timeout does not apply to waits for table locks.

The lock wait timeout value does not apply to deadlocks when innodb_deadlock_detect is enabled (the default) because InnoDB detects deadlocks immediately and rolls back one of the deadlocked transactions. When innodb_deadlock_detect is disabled, InnoDB relies on innodb_lock_wait_timeout for transaction rollback when a deadlock occurs. See Section 17.7.5.2, “Deadlock Detection”.

innodb_lock_wait_timeout can be set at runtime with the SET GLOBAL or SET SESSION statement. Changing the GLOBAL setting requires privileges sufficient to set global system variables (see Section 7.1.9.1, “System Variable Privileges”) and affects the operation of all clients that subsequently connect. Any client can change the SESSION setting for innodb_lock_wait_timeout, which affects only that client.

innodb_log_buffer_size

The size in bytes of the buffer that InnoDB uses to write to the log files on disk. The default is 16MB. A large log buffer enables large transactions to run without the need to write the log to disk before the transactions commit. Thus, if you have transactions that update, insert, or delete many rows, making the log buffer larger saves disk I/O. For related information, see Memory Configuration, and Section 10.5.4, “Optimizing InnoDB Redo Logging”. For general I/O tuning advice, see Section 10.5.8, “Optimizing InnoDB Disk I/O”.

innodb_log_checkpoint_fuzzy_now

Enable this debug option to force InnoDB to write a fuzzy checkpoint. This option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_log_checkpoint_now

Enable this debug option to force InnoDB to write a checkpoint. This option is only available if debugging support is compiled in using the WITH_DEBUG CMake option.

innodb_log_checksums

Enables or disables checksums for redo log pages.

innodb_log_checksums=ON enables the CRC-32C checksum algorithm for redo log pages. When innodb_log_checksums is disabled, the contents of the redo log page checksum field are ignored.

Checksums on the redo log header page and redo log checkpoint pages are never disabled.

innodb_log_compressed_pages

Specifies whether images of re-compressed pages are written to the redo log. Re-compression may occur when changes are made to compressed data.

innodb_log_compressed_pages is enabled by default to prevent corruption that could occur if a different version of the zlib compression algorithm is used during recovery. If you are certain that the zlib version is not subject to change, you can disable innodb_log_compressed_pages to reduce redo log generation for workloads that modify compressed data.

To measure the effect of enabling or disabling innodb_log_compressed_pages, compare redo log generation for both settings under the same workload. Options for measuring redo log generation include observing the Log sequence number (LSN) in the LOG section of SHOW ENGINE INNODB STATUS output, or monitoring Innodb_os_log_written status for the number of bytes written to the redo log files.

For related information, see Section 17.9.1.6, “Compression for OLTP Workloads”.

innodb_log_file_size

The size in bytes of each log file in a log group. The combined size of log files (innodb_log_file_size * innodb_log_files_in_group) cannot exceed a maximum value that is slightly less than 512GB. A pair of 255 GB log files, for example, approaches the limit but does not exceed it. The default value is 48MB.

Generally, the combined size of the log files should be large enough that the server can smooth out peaks and troughs in workload activity, which often means that there is enough redo log space to handle more than an hour of write activity. The larger the value, the less checkpoint flush activity is required in the buffer pool, saving disk I/O. Larger log files also make crash recovery slower.

The minimum innodb_log_file_size is 4MB.

For related information, see Redo Log Configuration. For general I/O tuning advice, see Section 10.5.8, “Optimizing InnoDB Disk I/O”.

If innodb_dedicated_server is enabled, the innodb_log_file_size value is automatically configured if it is not explicitly defined. For more information, see Section 17.8.12, “Enabling Automatic Configuration for a Dedicated MySQL Server”.

innodb_log_files_in_group

The number of log files in the log group. InnoDB writes to the files in a circular fashion. The default (and recommended) value is 2. The location of the files is specified by innodb_log_group_home_dir. The combined size of log files (innodb_log_file_size * innodb_log_files_in_group) can be up to 512GB.

For related information, see Redo Log Configuration.

If innodb_dedicated_server is enabled, innodb_log_files_in_group is automatically configured if it is not explicitly defined. For more information, see Section 17.8.12, “Enabling Automatic Configuration for a Dedicated MySQL Server”.

innodb_log_group_home_dir

The directory path to the InnoDB redo log files.

For related information, see Redo Log Configuration.

innodb_log_spin_cpu_abs_lwm

Defines the minimum amount of CPU usage below which user threads no longer spin while waiting for flushed redo. The value is expressed as a sum of CPU core usage. For example, The default value of 80 is 80% of a single CPU core. On a system with a multi-core processor, a value of 150 represents 100% usage of one CPU core plus 50% usage of a second CPU core.

For related information, see Section 10.5.4, “Optimizing InnoDB Redo Logging”.

innodb_log_spin_cpu_pct_hwm

Defines the maximum amount of CPU usage above which user threads no longer spin while waiting for flushed redo. The value is expressed as a percentage of the combined total processing power of all CPU cores. The default value is 50%. For example, 100% usage of two CPU cores is 50% of the combined CPU processing power on a server with four CPU cores.

The innodb_log_spin_cpu_pct_hwm variable respects processor affinity. For example, if a server has 48 cores but the mysqld process is pinned to only four CPU cores, the other 44 CPU cores are ignored.

For related information, see Section 10.5.4, “Optimizing InnoDB Redo Logging”.

innodb_log_wait_for_flush_spin_hwm

Defines the maximum average log flush time beyond which user threads no longer spin while waiting for flushed redo. The default value is 400 microseconds.

For related information, see Section 10.5.4, “Optimizing InnoDB Redo Logging”.

innodb_log_write_ahead_size

Defines the write-ahead block size for the redo log, in bytes. To avoid “read-on-write”, set innodb_log_write_ahead_size to match the operating system or file system cache block size. The default setting is 8192 bytes. Read-on-write occurs when redo log blocks are not entirely cached to the operating system or file system due to a mismatch between write-ahead block size for the redo log and operating system or file system cache block size.

Valid values for innodb_log_write_ahead_size are multiples of the InnoDB log file block size (2ⁿ). The minimum value is the InnoDB log file block size (512). Write-ahead does not occur when the minimum value is specified. The maximum value is equal to the innodb_page_size value. If you specify a value for innodb_log_write_ahead_size that is larger than the innodb_page_size value, the innodb_log_write_ahead_size setting is truncated to the innodb_page_size value.

Setting the innodb_log_write_ahead_size value too low in relation to the operating system or file system cache block size results in “read-on-write”. Setting the value too high may have a slight impact on fsync performance for log file writes due to several blocks being written at once.

For related information, see Section 10.5.4, “Optimizing InnoDB Redo Logging”.

innodb_log_writer_threads