From MariaDB 10.5, InnoDB uses asynchronous I/O to read from and write to disk asynchronously. This forms part of the InnoDB Background Thread Pool.
Each asynchronous IO operation goes through multiple stages:
SUBMITTED – The IO operation is initiated.
For asynchronous writes, this typically occurs in the buffer pool flushing code.
For asynchronous reads, this may happen during buffer pool loading at startup or in prefetching logic.
COMPLETED_IN_OS – The operating system notifies InnoDB that the I/O operation is complete.
If using libaio or io_uring, a dedicated thread handles this notification.
The completed IO operation is then submitted to InnoDB’s internal thread pool (tpool).
EXECUTING_COMPLETION_TASK – A tpool thread processes the completion task for the IO operation.
COMPLETED – The IO operation is fully handled.
The total number of pending asynchronous IO operations is limited by:
total_count = number_of_IO_threads * 256where number_of_IO_threads refers to either innodb_io_read_threads or innodb_io_write_threads.
Each IO operation is associated with an IO slot, which contains necessary metadata such as the file handle, operation type, offset, length, and any OS error codes. Initially, all total_count slots are free, but as pending IO requests accumulate, slots get occupied. If all slots are in use, additional IO requests must wait for a free slot.
The number of completion tasks (EXECUTING_COMPLETION_TASK stage) that can run in parallel is also limited by innodb_io_read_threads or innodb_io_write_threads. If too many IO operations complete simultaneously, they cannot all be processed in parallel and must be queued, respecting the thread limit.
From MariaDB 11.5, a number of status variables were added to give insight into the above operations:
Similar variables exist for write operations:
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