MySQL :: MySQL 5.5 Reference Manual :: 14.9.1 InnoDB Startup Configuration

Specifying Options in a MySQL Configuration File

Because MySQL uses data file and log file configuration settings to initialize the InnoDB instance, it is recommended that you define these settings in a configuration file that MySQL reads at startup, prior to initializing InnoDB for the first time. InnoDB is initialized when the MySQL server is started, and the first initialization of InnoDB normally occurs the first time you start the MySQL server.

You can place InnoDB options in the [mysqld] group of any option file that your server reads when it starts. The locations of MySQL option files are described in Section 4.2.6, “Using Option Files”.

To make sure that mysqld reads options only from a specific file, use the --defaults-file option as the first option on the command line when starting the server:

mysqld --defaults-file=path_to_configuration_file

Viewing InnoDB Initialization Information

To view InnoDB initialization information during startup, start mysqld from a command prompt. When mysqld is started from a command prompt, initialization information is printed to the console.

For example, on Windows, if mysqld is located in C:\Program Files\MySQL\MySQL Server 5.5\bin, start the MySQL server like this:

C:\> "C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld" --console

On Unix-like systems, mysqld is located in the bin directory of your MySQL installation:

sell> bin/mysqld --user=mysql &

If you do not send server output to the console, check the error log after startup to see the initialization information InnoDB printed during the startup process.

For information about starting MySQL using other methods, see Section 2.10.5, “Starting and Stopping MySQL Automatically”.

Important Storage Considerations

Review the following storage-related considerations before proceeding with your startup configuration.

In some cases, database performance improves if the data is not all placed on the same physical disk. Putting log files on a different disk from data is very often beneficial for performance. For example, you can place system tablespace data files and log files on different disks. You can also use raw disk partitions (raw devices) for InnoDB data files, which may speed up I/O. See Section 14.10.3, “Using Raw Disk Partitions for the System Tablespace”.
InnoDB is a transaction-safe (ACID compliant) storage engine for MySQL that has commit, rollback, and crash-recovery capabilities to protect user data. However, it cannot do so if the underlying operating system or hardware does not work as advertised. Many operating systems or disk subsystems may delay or reorder write operations to improve performance. On some operating systems, the very fsync() system call that should wait until all unwritten data for a file has been flushed might actually return before the data has been flushed to stable storage. Because of this, an operating system crash or a power outage may destroy recently committed data, or in the worst case, even corrupt the database because of write operations having been reordered. If data integrity is important to you, perform some “pull-the-plug” tests before using anything in production. On OS X 10.3 and higher, InnoDB uses a special fcntl() file flush method. Under Linux, it is advisable to disable the write-back cache.
On ATA/SATA disk drives, a command such hdparm -W0 /dev/hda may work to disable the write-back cache. Beware that some drives or disk controllers may be unable to disable the write-back cache.
With regard to InnoDB recovery capabilities that protect user data, InnoDB uses a file flush technique involving a structure called the doublewrite buffer, which is enabled by default (innodb_doublewrite=ON). The doublewrite buffer adds safety to recovery following a crash or power outage, and improves performance on most varieties of Unix by reducing the need for fsync() operations. It is recommended that the innodb_doublewrite option remains enabled if you are concerned with data integrity or possible failures. For additional information about the doublewrite buffer, see Section 14.15.1, “InnoDB Disk I/O”.
If reliability is a consideration for your data, do not configure InnoDB to use data files or log files on NFS volumes. Potential problems vary according to OS and version of NFS, and include such issues as lack of protection from conflicting writes, and limitations on maximum file sizes.

System Tablespace Data File Configuration

System tablespace data files are configured using the innodb_data_file_path and innodb_data_home_dir configuration options.

The innodb_data_file_path configuration option is used to configure the InnoDB system tablespace data files. The value of innodb_data_file_path should be a list of one or more data file specifications. If you name more than one data file, separate them by semicolon (;) characters:

innodb_data_file_path=datafile_spec1[;datafile_spec2]...

For example, the following setting explicitly creates a minimally sized system tablespace:

[mysqld]
innodb_data_file_path=ibdata1:12M:autoextend

This setting configures a single 12MB data file named ibdata1 that is auto-extending. No location for the file is given, so by default, InnoDB creates it in the MySQL data directory.

Sizes are specified using K, M, or G suffix letters to indicate units of KB, MB, or GB.

A tablespace containing a fixed-size 50MB data file named ibdata1 and a 50MB auto-extending file named ibdata2 in the data directory can be configured like this:

[mysqld]
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend

The full syntax for a data file specification includes the file name, its size, and several optional attributes:

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

The autoextend and max attributes can be used only for the last data file in the innodb_data_file_path line.

If you specify the autoextend option for the last data file, InnoDB extends the data file if it runs out of free space in the tablespace. The increment is 64MB at a time by default. To modify the increment, change the innodb_autoextend_increment system variable.

If the disk becomes full, you might want to add another data file on another disk. For tablespace reconfiguration instructions, see Section 14.10.1, “Resizing the InnoDB System Tablespace”.

InnoDB is not aware of the file system maximum file size, so be cautious on file systems where the maximum file size is a small value such as 2GB. To specify a maximum size for an auto-extending data file, use the max attribute following the autoextend attribute. Use the max attribute only in cases where constraining disk usage is of critical importance, because exceeding the maximum size causes a fatal error, possibly including a crash. The following configuration permits ibdata1 to grow up to a limit of 500MB:

[mysqld]
innodb_data_file_path=ibdata1:12M:autoextend:max:500M

InnoDB creates tablespace files in the MySQL data directory by default (datadir). To specify a location explicitly, use the innodb_data_home_dir option. For example, to create two files named ibdata1 and ibdata2 in a directory named /myibdata, configure InnoDB like this:

[mysqld]
innodb_data_home_dir = /myibdata
innodb_data_file_path=ibdata1:50M;ibdata2:50M:autoextend

Note

InnoDB does not create directories, so make sure that the /myibdata directory exists before you start the server. Use the Unix or DOS mkdir command to create any necessary directories.

Make sure that the MySQL server has the proper access rights to create files in the data directory. More generally, the server must have access rights in any directory where it needs to create data files.

InnoDB forms the directory path for each data file by textually concatenating the value of innodb_data_home_dir to the data file name, adding a path name separator (slash or backslash) between values if necessary. If the innodb_data_home_dir option is not specified in my.cnf at all, the default value is the “dot” directory ./, which means the MySQL data directory. (The MySQL server changes its current working directory to its data directory when it begins executing.)

If you specify innodb_data_home_dir as an empty string, you can specify absolute paths for the data files listed in the innodb_data_file_path value. The following example is equivalent to the preceding one:

[mysqld]
innodb_data_home_dir =
innodb_data_file_path=/ibdata/ibdata1:50M;/ibdata/ibdata2:50M:autoextend

InnoDB Log File Configuration

By default, InnoDB creates two 5MB log files in the MySQL data directory (datadir) named ib_logfile0 and ib_logfile1.

The following options can be used to modify the default configuration:

innodb_log_group_home_dir defines directory path to the InnoDB log files (the redo logs). If this option is not configured, InnoDB log files are created in the MySQL data directory (datadir).
You might use this option to place InnoDB log files in a different physical storage location than InnoDB data files to avoid potential I/O resource conflicts. For example:
```
[mysqld]
innodb_log_group_home_dir = /dr3/iblogs
```
Note

InnoDB does not create directories, so make sure that the log directory exists before you start the server. Use the Unix or DOS mkdir command to create any necessary directories.
Make sure that the MySQL server has the proper access rights to create files in the log directory. More generally, the server must have access rights in any directory where it needs to create log files.
innodb_log_files_in_group defines the number of log files in the log group. The default and recommended value is 2.
innodb_log_file_size defines the size in bytes of each log file in the 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 4GB. A pair of 2047 MB log files, for example, approaches the limit but does not exceed it. The default log file size is 5MB. 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 needed in the buffer pool, saving disk I/O. For additional information, see Section 8.5.3, “Optimizing InnoDB Redo Logging”.

InnoDB Memory Configuration

MySQL allocates memory to various caches and buffers to improve performance of database operations. When allocating memory for InnoDB, always consider memory required by the operating system, memory allocated to other applications, and memory allocated for other MySQL buffers and caches. For example, if you use MyISAM tables, consider the amount of memory allocated for the key buffer (key_buffer_size). For an overview of MySQL buffers and caches, see Section 8.12.4.1, “How MySQL Uses Memory”.

Buffers specific to InnoDB are configured using the following parameters:

innodb_buffer_pool_size defines size of the buffer pool, which is the memory area that holds cached data for InnoDB tables, indexes, and other auxiliary buffers. The size of the buffer pool is important for system performance, and it is typically recommended that innodb_buffer_pool_size is configured to 50 to 75 percent of system memory. The default buffer pool size is 128MB. For additional guidance, see Section 8.12.4.1, “How MySQL Uses Memory”. For information about how to configure InnoDB buffer pool size, see Configuring InnoDB Buffer Pool Size. Buffer pool size can be configured at startup.
On systems with a large amount of memory, you can improve concurrency by dividing the buffer pool into multiple buffer pool instances. The number of buffer pool instances is controlled by the by innodb_buffer_pool_instances option. By default, InnoDB creates one buffer pool instance. The number of buffer pool instances can be configured at startup. For more information, see Section 14.9.2.2, “Configuring Multiple Buffer Pool Instances”.
innodb_additional_mem_pool_size defines size in bytes of a memory pool InnoDB uses to store data dictionary information and other internal data structures. The more tables you have in your application, the more memory you allocate here. If InnoDB runs out of memory in this pool, it starts to allocate memory from the operating system and writes warning messages to the MySQL error log. The default value is 8MB.
innodb_log_buffer_size defines the size in bytes of the buffer that InnoDB uses to write to the log files on disk. The default size is 8MB. A large log buffer enables large transactions to run without a need to write the log to disk before the transactions commit. If you have transactions that update, insert, or delete many rows, you might consider increasing the size of the log buffer to save disk I/O. innodb_log_buffer_size can be configured at startup. For related information, see Section 8.5.3, “Optimizing InnoDB Redo Logging”.

Warning

On 32-bit GNU/Linux x86, be careful not to set memory usage too high. glibc may permit the process heap to grow over thread stacks, which crashes your server. It is a risk if the memory allocated to the mysqld process for global and per-thread buffers and caches is close to or exceeds 2GB.

A formula similar to the following that calculates global and per-thread memory allocation for MySQL can be used to estimate MySQL memory usage. You may need to modify the formula to account for buffers and caches in your MySQL version and configuration. For an overview of MySQL buffers and caches, see Section 8.12.4.1, “How MySQL Uses Memory”.

innodb_buffer_pool_size
+ key_buffer_size
+ max_connections*(sort_buffer_size+read_buffer_size+binlog_cache_size)
+ max_connections*2MB

Each thread uses a stack (often 2MB, but only 256KB in MySQL binaries provided by Oracle Corporation.) and in the worst case also uses sort_buffer_size + read_buffer_size additional memory.

On Linux, if the kernel is enabled for large page support, InnoDB can use large pages to allocate memory for its buffer pool and additional memory pool. See Section 8.12.4.2, “Enabling Large Page Support”.