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
