What is fsync used for?

Oh, and by the way: according to POSIX, The fsync() function is intended to force a physical write of data from the buffer cache, and to assure that after a system crash or other failure that all data up to the time of the fsync() call is recorded on the disk.

Is fsync necessary?

In your case where you open-write-close you don’t need to fsync . The OS knows which parts of the file are not yet written to the device. So if a second process wants to read the file the OS knows that it has the file content in memory and will not read the file content from the device.

What does fsync return?

RETURN VALUE Upon successful completion, fsync() returns 0. Otherwise, -1 is returned and errno is set to indicate the error. If the fsync() function fails, outstanding I/O operations are not guaranteed to have been completed.

Is fsync expensive?

The fsync call is very expensive, but it is essential to databases as it allows for durability (the “D” of the ACID acronym).

Is fsync Atomic?

The contract of fsync() is that when a call to fsync() returns, then all file modifications have been written to the actual disk. Therefore, after an fsync() , power loss is no longer an issue. Sadly, fsync() is not atomic itself.

Does Fflush call fsync?

But is that the case: is there a call to fsync from fflush? No, calling fflush on a POSIX system does not imply that fsync will be called.

What is fsync in PostgreSQL?

If this option is on, the PostgreSQL server will try to make sure that updates are physically written to disk, by issuing fsync() system calls or various equivalent methods (see wal_sync_method). This ensures that the database cluster can recover to a consistent state after an operating system or hardware crash.

What is InnoDB fsync?

FSYNC: Data and log files are opened with no options, and the fsyncsystem call is used when the engine requires flushing the data and log files. This option causes double buffering: page cache. InnoDB buffer pools. O_DSYNC: O_SYNC: This flag is used to open the log files while the data files are opened with no options.

Are writes Atomic?

Technically, yes: future reads must return the entire contents of the write, or none of it. […] Unfortunately, that writes are atomic in general is not what POSIX is saying and even if POSIX tried to say it, it’s extremely likely that no Unix system would actually comply and deliver fully atomic writes.

Is file write Atomic?

Firstly, O_APPEND or the equivalent FILE_APPEND_DATA on Windows means that increments of the maximum file extent (file “length”) are atomic under concurrent writers. This is guaranteed by POSIX, and Linux, FreeBSD, OS X and Windows all implement it correctly.

https://www.youtube.com/watch?v=OvAoag2MODo