The sync system call forces a write of dirty (modified) buffers in the block buffer cache out to disk. The kernel keeps this information in core to reduce the number of disk I/O transfers required by the system. As information in the cache is lost after a system crash, a sync system call is issued frequently by the user process syncer(4) (about every 30 seconds).
The fsync(2) system call may be used to synchronize individual file descriptor attributes.
fsync(2), syncer(4), sync(8)