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This article introduces the relevant knowledge of "what is the role of innodb_flush_method function in MySQL". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

The parameter innodb_flush_method controls the opening and writing mode of the innodb data file and redo log. For this parameter, it is described as follows:

There are three values: fdatasync (default), Olympus DSYNCrect

The default is fdatasync. Call fsync () to brush the data file and buffer of redo log.

When O_DSYNC, innodb uses O_SYNC to open and write redo log, and fsync () to write data files

When O_DIRECT, innodb uses O_DIRECT to open the data file, and fsync () to swipe the data file and redo log

First of all, the write operation of the file includes three steps: open,write,flush

The fsync (int fd) function most often mentioned above is used to flush the buffer related to the file referred to in the fd file descriptor to disk, and flush completes the metadata information (such as modification date, creation date, etc.) before flush is successful.

Opening the redo file with O_SYNC means that when the write log is logged, the data is write to disk and the metadata needs to be updated before the success is returned.

O_DIRECT means that our write operation is written directly from mysql innodb buffer to disk.

At this point, I would like to summarize the three ways to write data:

Fdatasync mode: when writing data, the write step does not need to be actually written to disk (it may be written to the operating system buffer and the completion will be returned). The real completion is the flush operation, and the buffer is handed over to the operating system to flush, and the metadata information of the file needs to be updated to disk.

O_DSYNC mode: log writing is done at write step, while data file writing is done through fsync at flush step.

O_DIRECT mode: the data file is written directly from mysql innodb buffer to disk, and does not need to be buffered by the operating system. The real completion is at the step of flush, and the log is buffered by OS.

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