Movatterモバイル変換

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 theinnodb_use_native_aio configuration option, which applies to Linux systems only and is enabled by default. On other Unix-like systems,InnoDB uses synchronous I/O only. Historically,InnoDB only used asynchronous I/O on Windows systems. Using the asynchronous I/O subsystem on Linux requires thelibaio library.

With synchronous I/O, query threads queue I/O requests, andInnoDB background threads retrieve the queued requests one at a time, issuing a synchronous I/O call for each. When an I/O request is completed and the I/O call returns, theInnoDB background thread that is handling the request calls an I/O completion routine and returns to process the next request. The number of requests that can be processed in parallel isn, wheren is the number ofInnoDB background threads. The number ofInnoDB background threads is controlled byinnodb_read_io_threads andinnodb_write_io_threads. SeeSection 17.8.5, “Configuring the Number of Background InnoDB I/O Threads”.

With native AIO, query threads dispatch I/O requests directly to the operating system, thereby removing the limit imposed by the number of background threads.InnoDB background threads wait for I/O events to signal completed requests. When a request is completed, a background thread calls an I/O completion routine and resumes waiting for I/O events.

The advantage of native AIO is scalability for heavily I/O-bound systems that typically show many pending reads and writes inSHOW ENGINE INNODB STATUS output. The increase in parallel processing when using native AIO means that the type of I/O scheduler or properties of the disk array controller have a greater influence on I/O performance.

A potential disadvantage of native AIO for heavily I/O-bound systems is lack of control over the number of I/O write requests dispatched to the operating system at once. Too many I/O write requests dispatched to the operating system for parallel processing could, in some cases, result in I/O read starvation, depending on the amount of I/O activity and system capabilities.

If a problem with the asynchronous I/O subsystem in the OS preventsInnoDB from starting, you can start the server withinnodb_use_native_aio=0. This option may also be disabled automatically during startup ifInnoDB detects a potential problem such as a combination oftmpdir location,tmpfs file system, and Linux kernel that does not support asynchronous I/O ontmpfs.