MySQL 8.4 Release Notes
17.11.1 InnoDB 磁盘 I/O
InnoDB uses asynchronous disk I/O where possible, by creating a number of threads to handle I/O operations, while permitting other database operations to proceed while the I/O is still in progress. On Linux and Windows platforms, InnoDB uses the available OS and library functions to perform “native” asynchronous I/O. On other platforms, InnoDB still uses I/O threads, but the threads may actually wait for I/O requests to complete; this technique is known as “simulated” asynchronous I/O.
If InnoDB can determine there is a high probability that data might be needed soon, it performs read-ahead operations to bring that data into the buffer pool so that it is available in memory. Making a few large read requests for contiguous data can be more efficient than making several small, spread-out requests. There are two read-ahead heuristics in InnoDB:
-
In sequential read-ahead, if
InnoDBnotices that the access pattern to a segment in the tablespace is sequential, it posts in advance a batch of reads of database pages to the I/O system. -
In random read-ahead, if
InnoDBnotices that some area in a tablespace seems to be in the process of being fully read into the buffer pool, it posts the remaining reads to the I/O system.
For information about configuring read-ahead heuristics, see Section 17.8.3.4, “Configuring InnoDB Buffer Pool Prefetching (Read-Ahead)”.
双写缓冲区
InnoDB uses a novel file flush technique involving a structure called the doublewrite buffer, which is enabled by default in most cases (innodb_doublewrite=ON). It adds safety to recovery following an unexpected exit or power outage, and improves performance on most varieties of Unix by reducing the need for fsync() operations.
Before writing pages to a data file, InnoDB first writes them to a storage area called the doublewrite buffer. Only after the write and the flush to the doublewrite buffer has completed does InnoDB write the pages to their proper positions in the data file. If there is an operating system, storage subsystem, or unexpected mysqld process exit in the middle of a page write (causing a torn page condition), InnoDB can later find a good copy of the page from the doublewrite buffer during recovery.
For more information about the doublewrite buffer, see Section 17.6.4, “Doublewrite Buffer”.