[stefanha]

The Linux RWF_DSYNC flag sets the Full Unit Access (FUA) bit in write requests. This can be used instead of fdatasync(2) in some cases. It only syncs a specific write request instead of the entire disk write cache.

[zozbot234]

You should prefer RWF_SYNC in case the write involves changes to the file metadata (For example, most append operations will alter the file size).

[stefanha]

Agreed, when metadata changes are involved then RWF_SYNC must be used.

RWF_DSYNC is sufficient and faster when data is overwritten without metadata changes to the file.

[LtdJorge]

Not really, RWF_DSYNC is equivalent to open(2) with O_DSYNC when writing which is equivalent to write(2) followed by fdatasync(2) and:

  fdatasync() is similar to fsync(), but does not flush modified
       metadata unless that metadata is needed in order to allow a
       subsequent data retrieval to be correctly handled.  For example,
       changes to st_atime or st_mtime (respectively, time of last access
       and time of last modification; see inode(7)) do not require
       flushing because they are not necessary for a subsequent data read
       to be handled correctly.  On the other hand, a change to the file
       size (st_size, as made by say ftruncate(2)), would require a
       metadata flush.