Linux sync Io:sync, Fsync and Fdatasync

"Linux sync Io:sync, Fsync and Fdatasync"

Traditional UNIX implementations have a buffer cache or page cache in the kernel, and most disk I/O is buffered. When writing data to a file, the kernel usually copies the data into one of the buffers, and if the buffer is not yet full, it is not queued to the output queue, but waits for it to be full, or when the kernel needs to reuse the buffer to hold other disk block data, then the buffer is queued to the output queue, and then when it arrives at the first To perform the actual I/O operation. This type of output is called deferred write (delayed write)

Deferred write reduces disk read and write times, but reduces file content updates so that data written to the file is not written to disk for a period of time. In the event of a system failure, this delay may result in the loss of file update content. To ensure consistency between the actual file system on the disk and the content in the buffer cache, the UNIX system provides sync, Fsync, and Fdatasync three functions.

The sync function simply queues all the modified block buffers into the write queue and then returns, not waiting for the actual write disk operation to end.

The system daemon, commonly referred to as update, periodically calls the sync function (typically every 30 seconds). This ensures that the block buffers of the kernel are flushed periodically. Command Sync (1) also calls the Sync function.
The Fsync function works only on a single file specified by the file descriptor Filedes, and waits for the write disk operation to end and then returns. Fsync can be used in applications such as databases, which need to ensure that modified blocks are immediately written to disk.
The Fdatasync function is similar to Fsync, but it affects only the data portion of the file. In addition to the data, Fsync also synchronizes the properties of the updated file.

Reference: http://blog.csdn.net/cywosp/article/details/8767327

For a database that provides transactional support, when a transaction commits, it is necessary to ensure that the transaction log (which contains the modification operation and a commit record) is fully written to the hard disk before the transaction is committed and returned to the application tier.

A simple question: on the *nix operating system, how to ensure that the updated content of the file is successfully persisted to the hard disk?

Linux sync Io:sync, Fsync and Fdatasync