SYNC_FILE_RANGE_WAIT_AFTER blocks the calling process until the newly-initiated writes complete.
An application which wants to initiate writeback of all dirty pages should provide the first two flags. Providing all three flags guarantees that those pages are actually on disk when the call returns.
The new implementation avoids distorting the posix_fadvise() system call. It also allows synchronization operations to be performed with a single call, instead of the multiple calls required by the previous attempt. In the future, it may also be possible to add other operations to the flags list; the ability to request metadata synchronization seems to be high on the list.
(Thanks to Michael Kerrisk - who agitated for this change - for providing some of the background information).
也可以man sync_file_range下他的具体作用, 但是请注意，sync_file_range是不可移植的。
sync_file_range – sync a file segment with disk
sync_file_range() permits fine control when synchronising the open file referred to by the file descriptor fd with disk.
offset is the starting byte of the file range to be synchronised. nbytes specifies the length of the range to be synchronised, in bytes; if nbytes is zero, then all bytes from offset through to the end of file are synchronised. Synchronisation is in units of the system page size: offset is rounded down to a page boundary; (offset+nbytes-1) is rounded up to a page boundary.
sync_file_range可以让我们在做多个更新后，一次性的刷数据，这样大大提高IO的性能。 具体的实现在fs/sync.c里面，有兴趣的同学可以围观下。
著名的fio测试工具支持sync_file_range来做sync操作，我们在决定在我们的应用中使用该syscall之前不妨先fio测试一把。
祝大家玩的开心。