14.4 sync: Synchronize cached writes to persistent storage

sync synchronizes in memory files or file systems to persistent storage. Synopsis:

sync [option] [file]…

sync writes any data buffered in memory out to the storage device. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The sync program does nothing but exercise the sync, syncfs, fsync, and fdatasync system calls.

The kernel keeps data in memory to avoid doing (relatively slow) device reads and writes. This improves performance, but if the computer crashes, data may be lost or the file system corrupted as a result. The sync command instructs the kernel to write data in memory to persistent storage.

If any argument is specified then only those files will be synchronized using the fsync(2) syscall by default.

If at least one file is specified, it is possible to change the synchronization method with the following options. Also see Common options.

‘-d’ ¶

‘--data’

Use fdatasync(2) to sync only the data for the file, and any metadata required to maintain file system consistency.

‘-f’ ¶

‘--file-system’

Synchronize all the I/O waiting for the file systems that contain the file, using the syscall syncfs(2). You would usually not specify this option if passing a device node like ‘/dev/sda’ for example, as that would sync the containing file system rather than the referenced one. Depending on the system, passing individual device nodes or files may have different sync characteristics than using no arguments. I.e., arguments passed to fsync(2) may provide greater guarantees through write barriers, than a global sync(2) used when no arguments are provided.

An exit status of zero indicates success, and a nonzero value indicates failure.