sync_file_range - sync a file segment with disk (None of these operations writes out the file's metadata)

sync_file_range 可以将文件的部分范围作为目标，将对应范围内的脏页刷回磁盘，而不是整个文件的范围。

好处是，当我们对大文件进行了修改时，如果修改了大量的数据块，我们最后fsync的时候，可能会很慢。即使fdatasync，也是有问题的，例如这个大文件的长度在我们的修改过程中发生了变化，那么fdatasync将同时写metadata，而对于文件系统来说，单个文件系统的写metadata 是串行的，这势必导致影响其他用户操作metadata(如创建文件)。

sync_file_range是绝对不会写metadata的，所以用它非常合适，每次对文件做了小范围的修改时，立即调用sync_file_range，把对应的脏数据刷到磁盘，那么在结束对文件的修改后，再调用fdatasync (flush dirty data page), fsync(flush dirty data+metadata page)都是很块的。

sync_file_range的几个flag, 注意SYNC_FILE_RANGE_WRITE是异步的，所以如果你要达到以上目的话，那么最好不要使用异步模式，或者至少在调用fdatasync和fsync前，使用SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |

       SYNC_FILE_RANGE_WAIT_AFTER做一次全文件范围的sync_file_range。从而保证在调用fdatasync或fsync前，该文件的dirty page已经全部刷到磁盘了。

       SYNC_FILE_RANGE_WAIT_BEFORE

              Wait upon write-out of all pages in the specified range that

              have already been submitted to the device driver for write-out

              before performing any write.

       SYNC_FILE_RANGE_WRITE

              Initiate write-out of all dirty pages in the specified range

              which are not presently submitted write-out.  Note that even

              this may block if you attempt to write more than request queue

              size.

       SYNC_FILE_RANGE_WAIT_AFTER

              Wait upon write-out of all pages in the range after performing

              any write.

       Useful combinations of the flags bits are:

       SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE

              Ensures that all pages in the specified range which were dirty

              when sync_file_range() was called are placed under write-out.

              This is a start-write-for-data-integrity operation.

       SYNC_FILE_RANGE_WRITE

              Start write-out of all dirty pages in the specified range

              which are not presently under write-out.  This is an

              asynchronous flush-to-disk operation.  This is not suitable

              for data integrity operations.

       SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER)

              Wait for completion of write-out of all pages in the specified

              range.  This can be used after an earlier

              SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE operation

              to wait for completion of that operation, and obtain its

              result.

       SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |

       SYNC_FILE_RANGE_WAIT_AFTER

              This is a write-for-data-integrity operation that will ensure

              that all pages in the specified range which were dirty when

              sync_file_range() was called are committed to disk.

[参考]
1. http://man7.org/linux/man-pages/man2/sync_file_range.2.html