10.5.8 Optimizing InnoDB Disk I/O

If you follow best practices for database design and tuning techniques for SQL operations, but your database is still slow due to heavy disk I/O activity, consider these disk I/O optimizations. If the Unix top tool or the Windows Task Manager shows that the CPU usage percentage with your workload is less than 70%, your workload is probably disk-bound.

• Increase buffer pool size

  When table data is cached in the InnoDB buffer pool, it can be accessed repeatedly by queries without requiring any disk I/O. Specify the size of the buffer pool with the innodb_buffer_pool_size option. This memory area is important enough that it is typically recommended that innodb_buffer_pool_size is configured to 50 to 75 percent of system memory. For more information see, Section 10.12.3.1, “How MySQL Uses Memory”.

• Adjust the flush method

  In some versions of GNU/Linux and Unix, flushing files to disk with the Unix fsync() call and similar methods is surprisingly slow. If database write performance is an issue, conduct benchmarks with the innodb_flush_method parameter set to O_DSYNC.

• Configure a threshold for operating system flushes

  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.

• Use fdatasync() instead of fsync()

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

  A subset of innodb_flush_method settings such as fsync, O_DSYNC, and O_DIRECT use fsync() system calls. The innodb_use_fdatasync variable is applicable when using those settings.

• Use a noop or deadline I/O scheduler with native AIO on Linux

  InnoDB uses the asynchronous I/O subsystem (native AIO) on Linux to perform read-ahead and write requests for data file pages. This behavior is controlled by the innodb_use_native_aio configuration option, which is enabled by default. With native AIO, the type of I/O scheduler has greater influence on I/O performance. Generally, noop and deadline I/O schedulers are recommended. Conduct benchmarks to determine which I/O scheduler provides the best results for your workload and environment. For more information, see Section 17.8.6, “Using Asynchronous I/O on Linux”.

• Use direct I/O on Solaris 10 for x86_64 architecture

  When using the InnoDB storage engine on Solaris 10 for x86_64 architecture (AMD Opteron), use direct I/O for InnoDB-related files to avoid degradation of InnoDB performance. To use direct I/O for an entire UFS file system used for storing InnoDB-related files, mount it with the forcedirectio option; see mount_ufs(1M). (The default on Solaris 10/x86_64 is not to use this option.) To apply direct I/O only to InnoDB file operations rather than the whole file system, set innodb_flush_method = O_DIRECT. With this setting, InnoDB calls directio() instead of fcntl() for I/O to data files (not for I/O to log files).

• Use raw storage for data and log files with Solaris 2.6 or later

  When using the InnoDB storage engine with a large innodb_buffer_pool_size value on any release of Solaris 2.6 and up and any platform (sparc/x86/x64/amd64), conduct benchmarks with InnoDB data files and log files on raw devices or on a separate direct I/O UFS file system, using the forcedirectio mount option as described previously. (It is necessary to use the mount option rather than setting innodb_flush_method if you want direct I/O for the log files.) Users of the Veritas file system VxFS should use the convosync=direct mount option.

  Do not place other MySQL data files, such as those for MyISAM tables, on a direct I/O file system. Executables or libraries must not be placed on a direct I/O file system.

• Use additional storage devices

  Additional storage devices could be used to set up a RAID configuration. For related information, see Section 10.12.1, “Optimizing Disk I/O”.

  Alternatively, InnoDB tablespace data files and log files can be placed on different physical disks. For more information, refer to the following sections:

  • Section 17.8.1, “InnoDB Startup Configuration”

  • Section 17.6.1.2, “Creating Tables Externally”

  • Creating a General Tablespace

  • Section 17.6.1.4, “Moving or Copying InnoDB Tables”

• Consider non-rotational storage

  Non-rotational storage generally provides better performance for random I/O operations; and rotational storage for sequential I/O operations. When distributing data and log files across rotational and non-rotational storage devices, consider the type of I/O operations that are predominantly performed on each file.

  Random I/O-oriented files typically include file-per-table and general tablespace data files, undo tablespace files, and temporary tablespace files. Sequential I/O-oriented files include InnoDB system tablespace files, doublewrite files, and log files such as binary log files and redo log files.

  Review settings for the following configuration options when using non-rotational storage:

  • innodb_checksum_algorithm

    The crc32 option uses a faster checksum algorithm and is recommended for fast storage systems.

  • innodb_flush_neighbors

    Optimizes I/O for rotational storage devices. Disable it for non-rotational storage or a mix of rotational and non-rotational storage. It is disabled by default.

  • innodb_idle_flush_pct

    Permits placing a limit on page flushing during idle periods, which can help extend the life of non-rotational storage devices.

  • innodb_io_capacity

    The default setting of 10000 is generally sufficient.

  • innodb_io_capacity_max

    The default value of (2 * innodb_io_capacity) is intended for most workloads.

  • innodb_log_compressed_pages

    If redo logs are on non-rotational storage, consider disabling this option to reduce logging. See Disable logging of compressed pages.

  • innodb_log_file_size (deprecated)

    If redo logs are on non-rotational storage, configure this option to maximize caching and write combining.

  • innodb_redo_log_capacity

    If redo logs are on non-rotational storage, configure this option to maximize caching and write combining.

  • innodb_page_size

    Consider using a page size that matches the internal sector size of the disk. Early-generation SSD devices often have a 4KB sector size. Some newer devices have a 16KB sector size. The default InnoDB page size is 16KB. Keeping the page size close to the storage device block size minimizes the amount of unchanged data that is rewritten to disk.

  • binlog_row_image

    If binary logs are on non-rotational storage and all tables have primary keys, consider setting this option to minimal to reduce logging.

  Ensure that TRIM support is enabled for your operating system. It is typically enabled by default.

• Increase I/O capacity to avoid backlogs

  If throughput drops periodically because of InnoDB checkpoint operations, consider increasing the value of the innodb_io_capacity configuration option. Higher values cause more frequent flushing, avoiding the backlog of work that can cause dips in throughput.

• Lower I/O capacity if flushing does not fall behind

  If the system is not falling behind with InnoDB flushing operations, consider lowering the value of the innodb_io_capacity configuration option. Typically, you keep this option value as low as practical, but not so low that it causes periodic drops in throughput as mentioned in the preceding bullet. In a typical scenario where you could lower the option value, you might see a combination like this in the output from SHOW ENGINE INNODB STATUS:

  • History list length low, below a few thousand.

  • Insert buffer merges close to rows inserted.

  • Modified pages in buffer pool consistently well below innodb_max_dirty_pages_pct of the buffer pool. (Measure at a time when the server is not doing bulk inserts; it is normal during bulk inserts for the modified pages percentage to rise significantly.)

  • Log sequence number - Last checkpoint is at less than 7/8 or ideally less than 6/8 of the total size of the InnoDB log files.

• Store system tablespace files on Fusion-io devices

  You can take advantage of a doublewrite buffer-related I/O optimization by storing the files that contain the doublewrite storage area on Fusion-io devices that support atomic writes. (The doublewrite buffer storage area resides in doublewrite files. See Section 17.6.4, “Doublewrite Buffer”.) When doublewrite storage area files are placed on Fusion-io devices that support atomic writes, the doublewrite buffer is automatically disabled and Fusion-io atomic writes are used for all data files. 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.

  Note

  Because the doublewrite buffer setting is global, the doublewrite buffer is also disabled for data files that do not reside on Fusion-io hardware.

• Disable logging of compressed pages

  When using the InnoDB table compression feature, images of re-compressed pages are written to the redo log when changes are made to compressed data. This behavior is controlled by innodb_log_compressed_pages, which is enabled by default to prevent corruption that can 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, disable innodb_log_compressed_pages to reduce redo log generation for workloads that modify compressed data.